And the fact that there are (and always have been) so many of them equates to them being typical of pot smokers.
The stereotype is based in reality.
You obviously don’t know any pot smokers. Or, you don’t realize that many, many normal appearing, successful professional people are regular marijuana users.
You wouldn’t be able to identify them on sight. Much as you can’t identify everyone who uses alcohol based on the visible behaviours of some over-the-top out-of-control drunks.
Bigots don’t care much for facts, though.