Why are United fans so oblivious to importance of good managers?

It has always irked me that some United fans refuse to believe that managers have levels just like players and you simply can't win with average manager who has no plan and tactical nous. We've seen the likes of Klopp, Pep etc. build teams that play identically dominating brands of football across multiple leagues. It's as if they have some sort of formula, a secret recipe that if given time and adequate resources will definitely result in successful teams that dominate games and win...

Why are United fans so oblivious to importance of good managers?