It’s not out of solidarity, advertisers are worried they’ll turn into lightning rods: https://www.adweek.com/social-marketing/ripples-through-reddit-as-advertisers-weather-moderators-strike/
In short: money.
Long story is that a lot of these tech companies started as startups funded by VCs.
Borrowing money was cheap so they got dumped buckets of money onto them to burn in an effort to try to get a foothold and/or kill off competition by undercutting them.
Now that they've gained a foothold and in some cases have a near monopoly or duopoly and now that borrowing money isn't cheap anymore, they need to start cutting cost if not outright turn a profit.
And so the enshittification begins.
Specifically for Twitter, Musk needs to cut cost because he bought Twitter at a severe premium and has made it less valuable by the minute ever since he took over. This to the point that he is leaving bills unpaid.
Specifically for Reddit, they've burned through all that VC money and have been eying a juicy exit in the form of an IPO. An IPO would be a payday for everyone who initially invested into Reddit because now they can sell their shares for more than what they invested (or at least that's their hope). In order to get a good price once they go public they want to cut cost and increase revenue to seem as valuable as possible.
Specifically for YouTube, the ad game has been generating less and less revenue over time and advertisers have been burned in the past by having their ads placed next to objectionable content.
So the knee-jerk reaction is to severely tighten the rules for content, lest they be demonetized.
This however made creators realize that their livelihood in the form of the pittance that's called AdSense payout is very fragile, so they started moving to doing sponsorships, soliciting Patreon donations and partnering with Nebula.
Now YouTube is missing out on those revenue streams and often ad revenue as well as creators often turn off ads on their video when they have sponsor deals etc. So what does YouTube do? They started monetizing videos of creators who are not eligible for their partner program (i.e. place ads on videos and not share it with creators) and not give those creators the option to turn off ads, they started severely increasing the amount of ads on videos that do run ads, they started severely pushing YouTube Premium and now they're cracking down on adblockers.
Have you seen !rust@programming.dev?
Not a Rust dev but it looks pretty active to me.