I think it's rarely this simple, speaking as someone who actively self sabotaged through my first aborted attempt at college in a
much less prestigious institution than Harvard.
Maybe that's what's at the root of it, but it's rarely what these kids tell themselves, I don't think. Much more of a discovery about how delayed consequences are when you just ignore them, and how good it feels to focus on the random highs you can get over the satisfaction of a job well done that doesn't pay off for a long time.
Before you've ever skipped class, the presumption is the world ends were that to happen. Then you get so drunk you sleep through the alarm and... wait, nothing? Nobody cared?! You can just go get drunk, miss class, and your parents don't come out of the walls to beat you???
There are a lot of kids, I think, who've been driven through their academic lives via purely "stick" incentives. When they find out there isn't a stick looming over their head immediately, and there are a lot of distracting "carrots" within short reach, it's hard to come back from.