I've ben homeless. It's not fun. Nobody does it because they want to. Ending up on the street trying to make the most basic of normality work is really hard work. I didn't end up on meth or anything (I stuck to alcohol), but I understand why some people facing this do. When your life is utter shite, drugs help.
Without any kind of social safety net the people who fall out of the bottom of society have nowhere to go except this. Build a decent safety net and they won't be living in the park and the park becomes the better place you'd like it to be.
So, on behalf of the unhoused: sorry your kids can't play in the park but we're facing bigger problems. Helping us with our problems will help you with yours.