I think it's because for many years anecdotal reports from clinical psychologists and psychiatrists drove a narrative (convenient to pro-prohibition authorities) that cannabis caused psychosis. Large epidemiological studies now suggest that's probably not the case, but the original narrative is often parroted uncritically by news outlets and others.

There are almost certainly risks from taking cannabis, but they are almost certainly an order of magnitude smaller than the risks of drinking and smoking (individually... not just becase A and T are more widespread)

i) clinical psychologist