I think it's because we give managers too much power, especially signing players. This should have stopped and we should have learned from the Moyes debacle. Signing Fellaini when we actually needed a Kroos or Fabregas type midfielder, Moyes knew he needed a different profile of midfielder but instead of relying on club scouts or asking the club to pick a midfielder of similar mould, he went with Fellaini just for the heck of it.
LvG like the OP said really drilled down his philosophy into our squad. We again made the grave error of relying on him for transfers - Depay, Di Maria, Falcao, Schneiderlin. We had to pay dearly to get rid off these. This should have been where we wisened up and appointed someone who knows football to run the club instead of relying on managers. LvG with a competent DoF could have worked wonders and laid a foundation for modern football. We could have brought in modern managers to work on top of what LvG did with the squad.
We decided to rip it all up and got the moron Mourinho in. Again wasted transfer funds, started the downfall of Martial who looked like a good footballer till Mourinho came in.
I don't want to talk about the disaster that is OGS. Never deserved our job, started the idiotic signing British talent trend just because he couldn't attract any sort of talent from around the world.
I thought once we we appointed Rangnick in an advisory role and EtH as coach, we are finally turning things around. Then we got rid off Rangnick, did all the mistakes we did all over again - now ended up with Antony, Mount, Amrabat, Onana, Casemiro - the kind of footballers we shouldn't be considering. I mean there wasn't even talk of talent like Doku, Szoboslai on our radars who our rivals have easy signed but we have to go through nonsense transfer dramas like Mount's every fecking year.