It could be many things, but as a former D1 shooting coach I can tell you a lot of it is in game confusion. What I mean by that is when we correct form, it is easier to use a modified form when shooting lots of reps in practice, but in game shots often revert to the old form or something in between, and those are often really bad until the player makes the new shot instinctual. And some never do, which is why we don't often fully reform a shooters shot just make tweaks.
It is easier to learn good form when young, but unfortunately most systems I see encourage 3 pt shots at too early an age which encourages poor form as kids aren't generally strong enough to get these shots off with proper form.