For starters, thinking that players will just leave if a big offer comes in for them. Bruno has never been made available for transfer at United, and neither has he expressed any desire to leave. Add to that that he's been a well-paid and crucial player for a top 3 biggest club in the world, which club did you expect to come in with a huge bid for him? We aren't a stepping stone club, and players don't move to Manchester United for the short-term, unless they're well into their 30s and/or clearly aren't good enough to be a long-term player for us, like Weghorst or Ighalo. The only two clubs in the world that could've tempted him IMO are Barcelona and Real Madrid, but they haven't exactly been swimming in riches lately, and just because they haven't expressed serious interest in him, that doesn't mean he's not good enough for them. Bruno also joined just before Covid as well, and even the likes of Barca or Real wouldn't have been able to afford him in 2020 or 2021, for example. Bruno also hasn't expressed a desire to eventually leave, like Hazard, Rüdiger, Lewandowski, and other high profile signings the two Spanish giants have made in the last 5 years.
Why did no one try to buy Mbappé other than Real Madrid? Why has no one tried to get Rodri or any key player out of Man City? Why did no one bid for Mané until the player himself specifically told Liverpool that he wants to begin a new chapter in his career? It doesn't work like that. And I can mention many more examples. Leao seems content at Milan, hence there not being dozens of clubs offering 9 figures for him. De Jong at Barca, etc. Sporting wise, City are more attractive than us at the moment, but Manchester United aren't just a stepping stone club either, we've never been that. We're one of the biggest destinations for players in world football even in these last 11 years. Players also each have their own preferences. We're probably lucky that Bruno truly loves United and probably wouldn't be interested in the likes of PSG or Bayern Munich. Italian teams cannot really afford him. Going to United from Sporting isn't like going to Dortmund, or Tottenham, unless your ultimate goal as a player is Real Madrid or Barcelona. Bruno doesn't seem to be that guy, though.
If he did want to leave, there would suddenly be a lot of interest in him, I can guarantee you that.
The points you make regarding how he disrupts any kind of game-plan or that he's just having a purple patch is just baseless, so I won't go into that deeply.
Edit: You need to consider in the case of the two Spanish giants, that the only two big money transfer fees Madrid have paid in the last 4 years are Tchouameni for 80m euros and Bellingham for 103m. Mbappé's sign-on bonus will be the third time they truly splash the cash since Covid. Barcelona has been even more modest on the transfer market since 2020.