Reasons for not releasing bottom-line answers to old exam questions?
One, it's extra work, as pointed out by @NoName.

Two, the answer may not be as helpful as you think. Just a month ago, I was grading a stats and probability exam and for one of the questions students who made two very common errors (out of 200 exams, about 40 students made those mistakes) in their work got the right answer, because the errors 'cancelled'. If bottom line answers to that exam were released for the students next year, those making those two mistakes would mistakenly think they knew the correct solution.