I've personally worked in such a system. I can tell you -- as someone who asked the question often -- that often, the answer is "because we wrote it that way in 1975 and no one has bothered to spend the time to learn how to make it better since then.

It is not necessarily true something written forty years ago is right. It is not necessarily true that something written forty years ago has some secret logic in it that cannot be improved upon. It is not necessarily true that something written forty years ago cannot be improved by fresh eyes.

Obviously one must not change things recklessly -- but there are often improvements that can be made.