A number of reasons.
I think the narrative surrounding players shouldn’t be disregarded. By and large, our players are viewed as failures, as they were all brought in to help us win the league and we don’t. Sometimes we don’t even qualify for the Champions League. The conversation surrounding the club is always that of under achievement and that is the case with the players. In reality, these players are still better than, and finish above players viewed in a more positive light. But relative to expectation, all our players are seen as rubbish.
Someone like Maguire will have fans of any club outraged for going for him. He is seen as shit, but the reality is more a case of shit for United, not for West Ham. Those clubs want to sign promising players, while ours are all a bit damaged and judged.
If you compare that to City or Liverpool - their squads are seen as the standard, and clubs would love the opportunity to take even some of their fringe players who are title winners and simply unlucky to not be able to break into the best team in the league. Chelsea have a different model where they keep players on loan to retain value. But yea, even fringe players are seen as more valuable when you contribute to success rather than contribute to failure. Any player at Brighton will probably have a higher stock than those at United, despite United simply being better and finishing above them because Brighton are seen to have had an impressive season and not us. In fact, we are seen to be impressive this season, but even last season - we struggled to sell and were lambasted but still finished above teams like Brighton but just a different narrative.