Because there's only a few clubs who could be.
I think Nagelsmann wants/wanted a Frenkie type at Bayern, but they don't tend to spend that kind of money and are already bringing in Gravenberch for 20m as someone they can form into that player instead.
I'm sure PSG wouldn't mind him, but he does overlap quite a bit with Veratti.
Juve could obviously use him considering their midfield but they're going after the free Pogba instead.
It just leaves premier league sides, of which Chelsea have too many players of his ilk already (with De Jong falling somewhere in the middle of Jorginho, Kovacic, Kante in terms of skillset)
It basically just leaves City, who are the only ones I can see coming in for him (I think he'd be great alongside Rodri, but it would require change of shape by Pep) but as a club it seems like they only go after a player when it fits exactly into some plan/place they have for him, rather than simply because the opportunity's there (Although maybe not so much with Grealish?)
But yeah you honestly have just one potential rival for his signature and that's a picky City.
In terms of other United rivals: he'd be a good replacement for Xhaka but why would he want to go to Arsenal right now? And why would he believe that Conte of all people would finally let him play his own game?
Edit: completely forgot Liverpool exists
would be an interesting player for Liverpool but only if for some reason they decided to move on from Thiago