A lot of people come from households where either A) their parents took care of everything for them or B) their parents didn't give a trout and were filthy. Either way you end up with kids who don't clean for crap once they're out on their own.
2/3s of my crappy room mates had mothers who literally did everything for them when they lived at home and they never took up the slack once they were out. All of their food was either frozen, out of a can, or fast food, and they always let junk piles grow of trash, dirty dishes, and general clutter. My last roomie's room just smurfing stank. He moved in around August with a suitcase full of clothes that I didn't see him wash until probably October and he didn't hang up until he had a girl over in January.