I would guess that the big difference between, say, political science on the one hand and management studies on the other hand is that few politicians and bureaucrats expect the political scientists to come up with ready-made solutions and rules-of-thumb.

Another interesting point (which you also make) is that a very old – pre-modern, in fact – tradition for writing books or tales on successful governance lives on in management studies. Dear old Machiavelli made sure that this tradition became discredited in politics.