Reply #2: Because they're taking up a seat that a competent student could be filling.

[Posteritatis]

Barring significant problems or a high-pressure graduate program it's pretty hard to flunk out of college these days in a way that isn't mostly or entirely self-inflicted. I have very, very little sympathy for "D for Diploma" students or similar types at the college level, where they've generally run out of excuses to know better. By that point

If they go to college to fuck around, then they're going to college for the wrong reason. If they go to college, don't fuck around, but join the square-root club consistently, then they're obviously lacking some fundamentals, in which case they should deal with those and then maybe reattempt college.