You've made it pretty clear that it's possible, if you're willing to jump through a bunch of hoops and are a programmer, to begin to have the privilege of an altruistic lifestyle. That is, the kind that will help thousands if not millions of persons to get higher quality of life.
'just move somewhere with low cost of living' - what about family and friends? non-profits are hiring in the middle of nowhere, I guess? what about most housing requiring a year-long contract? what about rural internet connectivity being a complete joke, and most cellular plans are by the GB?
'just take care of yourself and get a low deductible health insurance plan' - what about if you aren't healthy? what about if you have dependents? Where's the good care now that you're living away from cities? You're suggesting we move away from the best that humanity and technology has to offer?
'just cut down your expenses by 10x!' - ...you're joking, right? It's going to be really hard to live on $200/mo in any american city. Cost of living is going up everywhere, unless you're suggesting moving somewhere that's not growing. And wages have been stagnating for decades. See this post by Michael Church - https://www.quora.com/Why-do-software-engineers-make-so-much Only so much can be cut.
'just freelance! I did it, so can you' - times are changing, and that lifestyle simply isn't compatible with your average student coming out of our education systems. Also, freelancing suffers from being a chicken and egg kind of problem.
"If it doesn't make sense for a single person in the U.S. then who does it make sense for?"
Exactly. Hardly anyone. Our society is structured in such a way that it's very hard to be altruistic, compared to being selfish and seeking greater and greater pay through whatever job.
Have a social app idea? Here's $mil!! Uber for Z? $mil!! Oh it didn't work out? Ok, what else, what else...
Have a altruistic app idea that will improve the quality of life in a way that's hard to measure? Er haven't you heard? Not even schools can get funding. Teacher pay is frozen indefinitely.
Why does it take someone to be independently wealthy first, or live a lifestyle very much unlike the rest of the population, to be an effective altruistic in our society?