The system has as much interest in curing your depression as it does curing obesity. Long term, these conditions are very costly. It takes people out of peak condition, reduces labor pool, increases number of patients taking up resources at hospitals and doctors offices, and so on. A system which encourages these things is a failing one, as taxpayer costs would only continue to balloon.
Additionally, since the thread is about psilocybin, people would still be filling up prescriptions for those.
Also, self-medicating is also very costly. Not only to the system, as people often wind up in hospitals due to laced products or incorrect doses (see marijuana in a lot of states where dealers really aren't concerned about quality), but also to the individual. People are spending absurd amounts of money on vitamins and mushroom teas and all that in hopes it helps, but rarely is that the case due to these categories not being strictly regulated.