Because the board tries to build and maintain a squad that stays true to the club philosophy. If they feel a certain player doesn't fit in, they won't buy him. Coaches tend to come and go, players stay longer.
Look at your past 3 managers. All with a completely different philosophy. Moyes gets in, buys certain players. Moyes gets the sack, vG comes in and says "I can't work with this set players, buy me these and those". These and those are bought, vG gets the sack, Mou comes in, and says "I can't work with these players..." Rinse and repeat.
In consequence, your squad needs a major overhaul every time the manager changes.