Logistically that is impossible and makes no sense.
As for why there are so many young people homeless in SF, it's probably because SF is one of the most willing cities trying to solve a national problem by itself. We also have great weather (not too hot in the summer, not too cold in the winter). And probably a lot of generous people walking the streets.
It's really horrible what our national government has allowed to happen in our country and it's impractical for our city to hold the burden to solve this nation's problems.