Because career bureaucrats are the only way of running a large organizational arrangement that can even reach a semblance of "deciding all sorts of things". A few hundred representatives can't go at it alone. You get career bureaucrats in private enterprise too, of course, but the idea is that they should at least be kept on a short leash to whatever extent is feasible. That fails completely when you're dealing with an actual government at any scale bigger than a small village or HOA.