This is just from your personal experience.
I've got a large number of friends who smoke pot everyday. They're far from non-functioning.
Two of my closest developer buddies smoke all day long. Like, literally, all day long, smoking as soon as they wake up in the morning. That's about as extreme as you can get in the realm of the pothead.
They're fine. They've held their jobs for long periods, been promoted, given raises... all of the things you expect from a "functioning" member of society.
I don't really hang out with very many non-functioning member of society and I know a LOT of people who smoke pot... maybe you tend to hang out with a lot of non-functioning people for some reason? A rather larger percentage of people smoke pot around the world, so it would tend to cross a number of socioeconomic boundaries.
To me, this issue you have with perceiving that pot is making people in to listless sacks of flesh seems to be a personal problem and entirely subjective.