It's the Manchester United effect.
No one really wants to play there and they don't give a flying fuck. The culture is toxic. You then see across town their rivals, where ironically the same thing applies, but at least when it comes to football Guardiola and the City directors know how to squeeze whatever it takes from the players and leave their grievances outside the pitch.
Bernardo Silva for example has wanted out for years yet here he is renewing his deal
They know that there's not much higher they can go from there and how hard it is to be transferred, so they just get on with their job. Manchester United on the other hand is a graveyard, The only player that managed to show signs of quality there as a transfer is Bruno Fernandes, a guy that was overlooked in Serie A after stints with Sampdoria and Udinese. Everyone knew he was talented, but he couldn't even make the radar for the teams competing for CL spots in an age where Serie A were looking for bargains he went to the 3rd club of Portugal for less than 10m when we thought Vecino for 24m was a better idea.
They think that signing big names is a good thing, but big names hate it there and don't pan out as they'd wish. Ironically, neither does Manchester City sign such big names*. Haaland is possibly the biggest name they signed ever. The rest were just extremely good players that were looking for a step up. De Bruyne, Bernardo Silva, Ederson, Ruben Dias, Rodri.. none of them were called world class before Manchester City a season or two there. That's what Manchester United should be doing with all the money in the world that they have but instead they have this toxic climate where they throw money at random players and then expect "big name" players to play as they did in their prime, at better and more organized clubs.. Not gonna happen.
Even with Ferguson they didn't have luck with world class players and they either made names of players that arrived domestically, or imported up-and-coming players like Ruud van Nistelrooij (gambled for him after a terrible injury), Patrice Evra (we basically let him go there, he was pretty much ours), Vidic and De Gea.
I really wish I live to see Manchester United get relegated. That'd be great
* Only under Mancini because that was the way to get a relegation/mid-table team to competing levels. Aguero, Dzeko, Toure and a few key signings like Kompany, David Silva and Kolarov to name a few.