It's much easier to prove that something happens, than something doesn't happen.
There is a lot of evidence that suggests that taking psychedellics can lead to schizophrenia. To conclusively prove that that person wouldn't have gotten it without those drugs is extremely difficult.
While I 100% agree that we can't conclusively prove causation at this point, we do have solid evidence that there is a correlation, and that shouldn't be discounted given the research into psycheddelics is too immature to have a chance of proving anything past that.
Also, we already know that drug use can have permanent effects that presumably would not have manifested if the person had not taken drugs, unless you think that people who didn't see hallucinations before LSD and see hallucinations after LSD would have started seeing hallucinations anyway.
So I don't see why it would be unreasonable to think that drug use can trigger an onset of schizophrenia in an already predisposed person that would have not happened otherwise.
Legalization would be highly irresponsible until we understand why.