I kind of agree, but also think it's important to understand a few things about this:

• The US needs to keep its military-industrial complex active and technologically advanced at all times if it wants to be a military powerhouse. It can't decide to start it up whenever it wants because war machines have gotten too advanced. During WWII, it was easy to get the complex rolling because they just needed to churn out simple prop planes, tanks, rifles, and food. Now, they need stealth planes, laser-guided munitions, and high-tech chips.
• Because of the geography of the US and the geopolitical situation, it would likely fight a two-front war. If the US goes to war with a formidable power, said power would surely ally with another. The US will not just fight China alone. Russia and North Korea would join. Therefore, the US military needs to be large enough to fight knowing that by population, the US is much smaller. China has just over 4 times the population of the US.
• Having an overwhelmingly large and technologically advanced military serves as a deterrent. It's best to never go to war. It saves lives, economies, social institutions, etc. By having a decisively superior military, those that would consider starting a war avoid doing so.
• The Department of Defense and military-industrial complex is a huge jobs program anyway. Service members receive training and all sorts of benefits that support them and their dependents. Military production companies receive reliable government contracts that make their business ventures stable investments while employees receive relatively adequate pay. If the government did not fund those contracts, all those businesses would go out of business and everyone involved would have to find other means of sustenance.
• The US provides military defense and deterrence for more than just itself. It's practically the department of defense for most Pacific islands including Japan and the Philippines. It's also a necessary supporter of the EU and South Korea.

I'm not saying that I agree to the spending or that we shouldn't spend more on social welfare, but the solution is not obviously clear as just spending less on defense in my opinion.