Why? Because it's not a very accurate stereotype.
I do first aid at music festivals and volunteer at a free clinic that serves a homeless population. I see a lot of spun folks and folks in varying stages of "burned out" (to use the term from the stereotype). I'm still in EMT school, but I've had to work with a lot of folks having adverse reactions to these substances.
Meth and alcohol, simply being poor, childhood trauma, getting into a bad medical situation, losing a job and not finding one; all of those things will put folks into some bad places.
But most substance abuse, in my limited experience, comes down to various traumas folks get into and maybe some chemical predispositions.
That's not how most of the folks I know who have had very adverse psychedelic experiences have gotten into those states. And it is generally a very transitory state.
While there are long-term psychological effects from stuff like acid (and IME, MDMA, Ket, and Nitrous are worse physiologically than things like LSD, C2B, DMT, psilocybin, mescaline, etc), generally psychedelic substances lead people to doing less of those drugs over time and that has a very different effect than, say, alcohol and chronic poverty.
Propaganda about these things has a hard time taking hold in the face of most folks lived reality.