Really?
Majority of clubs have head coaches, not managers. The boards assesses to choose the candidate that they think can bring approach that fit the profile. Just look at Ajax for instance, beside just the brand of football, they can change head coaches and continue the core principle of the brand. De Boer version can be pragmatic (liken to LVG) and hard to stomach it's still based on the same core principle of total football. The (different) board means to keep to the core value of the brand via the change in head coach means different outlook to the football. There is such drastically change in core value leading to wholesale change and forever rebuild every time there is changes in head coach position.
Look at us, Moyes to LVG. Firesale. Price hiking when purchase. LVG to Mourinho. Same. Distinct brands of football. Players from the point of being coachable till they're supposed a seasoned pro, yet the team have no identity.
However, there is a common theme during all these changes: paying big for players especially those with names. Some transfer makes no sense: Mata for Moyes. Di Maria Falcao for LVG. Alexis for Mourinho came out of no where. Some players just randomly renewed their contract which lead to them being hard to moved on, and stick around: Rooney, Nani, Jones, Rojo... And that's Mourinho contract extension. The managers then all agreed that they felt hard done by some behind the scenes promise, questioning ambition/direction of the club.
One or two you can say it's a learning curve. Three, with Martial new contract, then it's a pattern.
And no, if the managers are solely responsible for purchasing wrong players, then the change in structure is needed, not just sack the managers, and rinse repeat. The talk of DOF has been here for years whenever this clubs is about to change managers, but disappears after a while like nothing happen. Yet the common denominator of all this failure was not questioned.