Here's a really good set of answers from three years back.

/u/john_andrew_smith101 does a good job of explaining just how hard it really is to "flunk out" of basic training. Failure to "get it" means you just get punished more, get recycled back to a new class, etc. Guys that would make poor front-line soldiers would get shuffled off to non-combat positions. Or sent off to combat anyway. Boot camp sucks. Nobody wants to be there longer than necessary.