It’s not just a popcult meme or trope, these indecently-low-hanging jeans are something I’ve seen many times in real life.
I can think of two explanations:
If you’re a janitor, repairman, plumber, etc., you have to do a lot of squatting and stooping and kneeling and crawling; loose unbelted pants are practical.
It started out as 1., but caught on and morphed into an unofficial uniform.
Any other possibilities?
It’s called “builder’s smile” here. 
Toolbelts. They weigh on the pants, pulling them down, especially when a belt is not worn.
If you’re asking why it’s been adopted by teenage boys as a uniform, I don’t know. But I often want to run up and urgently whisper “Excuse me, but your PANTS are falling down!” when I see them.
It’s been teenage to twenty-something girls here for the last five years (the boys are just starting to do it).
As far as the girls go, look I’m a red blooded man, but this IS NOT sexy. I only want to see somebody’s underwear in specific situations, and a shopping mall is not one of them. As for the boys, I can’t imagine how embarrassed I’d have felt if my pants looked like that at that age. OK, so styles have changed, but IT STILL LOOKS STUPID.
Heh. You just noticed this? Don’t move to Oakland.
The theory I hear most often, with respect to this “style feature” among the gangsta rap culture here, is that it is meant to be ugly and disrespectful in order to repudiate society, of which they want no part.
Yikes! I just realized that the sentence above could just as easily have been describing the punk look of my youth… that’s really weird. Cause I think the buttcrack thing looks stupid too.
It’s different, man… it just is, okay?
Thought it was a “prisoner’s crack” - no belt, so the trousers keep slipping down.
Oh, that I know. (My Dad used to be a public schoolteacher.) It’s called “housin’,” from jail, the “house,” where your clothes are never the right size and had better be too big than too small.
I heard that gays in jail wear their uniforms tight to show off their merchandise to prospective buyers.
For women, I think the crack = butt cleavage. For men…nah, let’s not go there.
Remember the SNL skit where Aykroyd plays the refrigerator repairman? “I’ll need you to sign here…” [pulls out pencil from crack].
Later reprised on The Nanny, to much hilarity. Fran Drescher and Dan Aykroyd and their spouses have a close friendship that goes back to when Fran & Dan starred in Dr. Detroit.
For some reason, I get this great catalog sometimes that’s for working men - contractors, plumbers, etc. They sell lots of things to avoid plumber’s crack - I believe one of their shirts is even called “Crack Spackle”? And comes in a little tub, as if it were really spackle for your ass crack? Anyway, there is a market evidently for longer shirts. 
Does anybody know the catalog I’m thinking of? It’s one of those catalogs that’s taller than it is wide. I was looking for it to maybe order Himself some more heatproof underwear.
ETA - Sorry. I promised I wouldn’t say “Himself” anymore because people hate me in the Pit for it. Promise this is my last one.
Oh dear, it’s got #2 on it.
I’m only just 6 feet tall, but for almost 20 years I’ve worn Tall shirts specifically so that I would not have to worry about them becoming tucked out. Glad to see I’m an innovator.
Someone pitted you over “Himself”? :rolleyes:
In the UK it’s “builder’s crack”.
Well, it was a mini-rant. Kind of hurt my feelings, though.
It is indeed the Duluth Trading Company. Now - “The Way Better Boxer” or “MN Performance Boxers”? God, I guess we might as well get married, huh?
And don’t forget the lovely Man Show invention of Crack Spackle!
An while not true of all tradesmen, many of them have large pot-bellies that prohibit them from wearing their pants up around their waists. Instead, they wear them down around their narrower hips and from there it isn’t very far to the crack.
On girls I don’t know which is worse, a little crack showing or muffin-top with little rolls of fat overhanging the top of the jeans.
You can get both - crack and muffin top. I work at a public library, and our computer chairs for the public are backless. My god, the things I’ve seen.
I’d have said “plumber’s smile” ~ same difference, I guess.
The “bike rack” is also facilitated by the gut, which tends to push it’s way over the top of the waist band if there’s a lot of bending an push the pants down onto the hips.