You can keep sacking managers but there's a cost in doing so. Pay off the manager, his assistants etc. Pay the compensation for the next guy and all his staff. Sell off the players that don't fit the new regime and buy the new ones that, hopefully, do. All of this costs more if you sack mid season.
You also have another's cost, which is that if you get the reputation for pulling the trigger too quickly, some managers won't consider working for you. You reduce your attractiveness to the guys that want to build a long term project, which is surely what we want too?