Because you don't want to have a reputation of booting an executive while he's at his peak.
They do that all the time in Soccer in Europe.
A new head (CEO/director/board) comes in the middle of the season, all the media surrounds the current manager asking him how he feels about his job in the middle of a pennant race, owner of the team comes and gives the manager a new 3 year contract.
During the offseason they release the manager if he didn't do well or keep him if he did do well.