Firstly, because marijuana is illegal its users don't advertise their use, so the only ones whose use becomes known to you are the goofy and lazy ones.
Secondly, it's at least as likely that any cause and effect relationship runs the opposite way: that people who are already goofy and lazy are likelier than average to turn to marijuana use.
Having reached adulthood in the mid-sixties when penalties were greater, people volunteered their use of marihootchie, as if the smell that permeated their clothes, hair, and lungs wasn't a dead give away. And the more powerful versions of the last few decades are even more aromatic.
Co-workers who stepped out for a J always thought they were pulling a fast one on the rest of us in much the same way the three vodka martini lunch crowd thought their breathe was sweet. And coincidentally work performance for both classes was sub-par and mistake ridden, standing in stark contrast to their delusional self image.
Thanks for Mr Natural recollections and the chuckles.
Keep on truckin' dude...hahahaha