That works fine for some kinds of questions, but not for questions like "prove this group is cyclic" or "prove this function is holomorphic" - in those cases there is no answer separate from the derivation.

And those questions tend to be the hardest and most prone to bullshittery by students. Many times in exams I made up proofs that made no sense and hoped no-one would notice.

Often when given the end result of a computation students just make up some nonsense integral or something and pretend they evaluated it, even if they didn't. Not having the answer immediately at least forces an honest attempt.

Let's not kid ourselves, we were all students at one time and we know students are lazy.