Affording therapy doesn't mean access to competent therapy. There are plenty of therapists who got into it because depressed, anxious people are everywhere and it's super easy to charge upper middle class people $150/session to give them the most basic mental health care advice in existence.

And for some people that's all they need, and that's fine. But if you have an eating disorder, or you're LGBT, or you're a minority, or you have PTSD or trauma you need to work through, or basically any "complex" mental health care need, it is HARD if not impossible to find a competent therapist that is in network, near you, and affordable. And there's no way to know if the therapist you're thinking of seeing is competent in those things at all until you go there, open yourself up, and suddenly maybe they say something extremely shitty that sours the whole experience and you have to start over again.

If I hadn't been able to go to my local LGBT center to get therapy, I don't know what would have happened with me. And it still wasn't cheap! You can't just go to therapy a couple of times and be done with it, I was prescribed 16 sessions (which is like, the basic default amount and they will extend it further if you need it), and if your copay is even $10 or $20 that is a lot of money to drop on something you're not positive is even going to help you.

Which like, isn't to say that heroin is a good alternative. It's just that when someone has a $20 in their pocket and they can either spend it on a single therapy session that may or may not be beneficial to them (or can even be retraumatizing) or they can spend it on a bag of heroin that will for sure get them high and make them feel better right now, it's not hard to see why some people are going to choose the latter.