The answer is simple, because he is under contract with the club, if another team wants to loan him, then they should accept United's demands, that's all.
United has no obligation to compromise on anything here, they can of course compromise but they are not obligated to do so.
Napoli accepted United's demands when they asked for Tuanzebe, Leeds made the club a great offer when they asked to get James permanently, this is the right way of doing business, when United want to get a player, the other clubs also get to make high demands, as this is their right, and United should also demand what they deem acceptable to them.