I mean, there just wasn't any realistic threat that we expected from Russia or China or such. We've got sensor networks that should be able to pick up any aircraft even being prepared, much less flying in from a long ways out, even if they did take off.

There's some accident-risk price to pay for readiness -- like, you can have accidents with weapons, and any time that weapons are floating around outside arsenals, there's at least some potential for them to go astray. And the more weapons systems you have in a "ready to engage" status, the more-twitchy it makes everyone else. Suppose we kept a couple thousand fighters armed and on the runway. That's gonna make some other countries twitchy that they have little time to react.

The "DEFCON level" is basically a slider that trades shorter response time for increased risk of things going wrong.

https://en.wikipedia.org/wiki/DEFCON

If we can, we keep it at low levels. Minimizes risk of accidents, avoids putting pressure on other parties.

During the Cuban Missile Crisis, we had it at an elevated level. That means that we can respond rapidly, and we're more-prepared to get hit with a major nuclear strike and still hit back as hard as possible. But it also...creates room for things to go rather badly, accidentally.

https://en.wikipedia.org/wiki/Cuban_Missile_Crisis

At 10:00 pm EDT the next day, the US raised the readiness level of Strategic Air Command (SAC) forces to DEFCON 2. For the only confirmed time in US history, B-52 bombers went on continuous airborne alert, and B-47 medium bombers were dispersed to various military and civilian airfields and made ready to take off, fully equipped, on 15 minutes' notice.[114] One-eighth of SAC's 1,436 bombers were on airborne alert, and some 145 intercontinental ballistic missiles stood on ready alert, some of which targeted Cuba.[115] Air Defense Command (ADC) redeployed 161 nuclear-armed interceptors to 16 dispersal fields within nine hours, with one third maintaining 15-minute alert status.[92] Twenty-three nuclear-armed B-52s were sent to orbit points within striking distance of the Soviet Union so it would believe that the US was serious.

As part of that, military aircraft were loaded with nuclear weapons, including a fleet of interceptors, and dispersed to civilian airports and airstrips to minimize the number that could be destroyed on the ground in the event of a nuclear attack from the Soviet Union.

Some of those airfields were -- not surprisingly -- not as secured against ground intrusions as military bases. At one point, a security guard saw a shadowy figure moving around the outskirts of one such civilian airfield, fired a burst at it from his submachine gun, but it made it over the fence and away. He hit his sabotage alarm. At that alert level, the presumption is that any detected sabotage attempt would be likely part of a preemptive strike, and doctrine dictated that the whole interceptor force get airborne and start heading towards the Soviet Union. They were rolling down the runways across the US when the sabotage alarm was cancelled -- upon further investigation of the traces left, it turned out that the figure was probably just a bear. But...a shit-ton of warplanes armed with (air-to-air, not strategic) nuclear weapons leaving the ground and heading towards the Soviet Union creates further potential for inadvertent escalation.

We had one incident, some years back, where the ground crew at an arsenal dicked up, loaded a bomber with live nukes rather than inert missiles, and the crew inadvertently flew to another airbase before the crew there checked, noticed that they had live nuclear weapons on their field, and started pushing red buttons.

https://en.wikipedia.org/wiki/United_States_military_nuclear_incident_terminology#Bent_Spear

An example of a Bent Spear incident occurred on the August 2007 flight of a B-52 bomber from Minot AFB to Barksdale AFB which mistakenly carried six cruise missiles with live nuclear warheads.[4]

Now, okay, those are extreme examples of risks -- a few F-16s armed with conventional weapons don't pose as much of a concern. But it does illustrate, I think, that there's a tradeoff involved. At the time, the risk of accidents was considered higher than the benefit from having a more-rapid response.

In any event, after 9/11, doctrine was revised, and the ready fighters are now kept armed. I'm not saying that the move was the right one. I'm just saying that there are real tradeoffs to be maintaining a high alert level. The USAF hadn't been told to expect to deal with a civilian aircraft in US airspace suddenly going hostile, so they hadn't structured their response system accordingly. The RuAF may or may not have made decisions about how to deal with civilian aircraft.

The Mathias Rust situation that someone else mentioned, as a I recall, dealt with Soviet doctrine where responses had been relaxed to help avoid accidental shootdowns, and that was part of how he made it to Red Square.

googles

Yeah: "The local air regiment near Pskov was on maneuvers and, due to inexperienced pilots' tendency to forget correct IFF designator settings, local control officers assigned all traffic in the area friendly status, including Rust.[5]"

https://en.wikipedia.org/wiki/Mathias_Rust

Rust disappeared from the Finnish air traffic radar near Espoo.[5] Control personnel presumed an emergency and a rescue effort was organized, including a Finnish Border Guard patrol boat. They found an oil patch near Sipoo where Rust had disappeared from radar observation, and conducted an underwater search but did not find anything.

Rust crossed the Baltic coastline over Estonia and turned towards Moscow. At 14:29 he appeared on Soviet Air Defence Forces (PVO) radar and, after failure to reply to an Identification Friend or Foe (IFF) signal, was assigned combat number 8255. Three Surface-to-air missile battalions of 54th Air Defence Corps tracked him for some time, but failed to obtain permission to launch missiles at him.[9] All air defences were readied and two interceptors were sent to investigate. At 14:48, near Gdov, MiG-23 pilot Senior Lieutenant A. Puchnin observed a white sport airplane similar to a Yakovlev Yak-12 and asked for permission to engage, but was denied.[5][10]

The fighters lost contact with Rust soon after this. While they were being directed back to him, he disappeared from radar near Staraya Russa. West German magazine Bunte speculated that he might have landed there for some time, noting that he changed his clothes during his flight and that he took too much time to fly to Moscow considering his airplane's speed and the weather conditions.

Air defence re-established contact with Rust's plane several times but confusion resulted from all of these events. The PVO system had shortly before been divided into several districts, which simplified management but created additional work for tracking officers at the districts' borders. The local air regiment near Pskov was on maneuvers and, due to inexperienced pilots' tendency to forget correct IFF designator settings, local control officers assigned all traffic in the area friendly status, including Rust.[5]

Near Torzhok there was a similar situation, as increased air traffic was created by a search and rescue operation. Rust, flying a slow propeller-driven aircraft, was confused with one of the helicopters participating with the operation. He was detected several more times and given false friendly recognition twice. Rust was considered as a domestic training airplane defying regulations, and was assigned the least priority by air defense.[5]

Around 19:00, Rust appeared above Moscow. He had initially intended to land in the Kremlin, but he reasoned that landing inside, hidden by the Kremlin walls, would have allowed the KGB to arrest him and deny the incident. Therefore, he changed his landing place to Red Square.[5] Dense pedestrian traffic did not allow him to land there either, so after circling about the square one more time, he was able to land on Bolshoy Moskvoretsky Bridge by St. Basil's Cathedral. A later inquiry found that trolleybus wires normally strung over the bridge—which would have prevented his landing there—had been removed for maintenance that morning, and were replaced the next day.[5] After taxiing past the cathedral, he stopped about 100 metres (330 ft) from the square, where he was greeted by curious passersby and asked for autographs.[11] When asked where he was from, he replied "Germany" making the bystanders think he was from East Germany; but when he said West Germany, they were surprised.[12] A British doctor videotaped Rust circling over Red Square and landing on the bridge.[12] Rust was arrested two hours later.[13]

Is it embarrassing? Well, I guess so. Rust made it to pretty sensitive airspace, shouldn't have. But, big picture...odds are also pretty good that if NATO's going to have a war with the Warsaw Pact, it's probably not going to involve sending a little prop plane to Red Square. Not saying that there's no risk there for a decapitation strike or something, but the Soviet airforce had to make a tradeoff in terms of how many of their own aircraft they shoot down accidentally versus whether they make sure to deal with some little prop plane wandering around.