Because it is in the interest of Juve to keep Pogba. And then sell him next year "for as much money as you can make", while being better prepared, if he wants to leave at all cost.
According to your logic we should immediately sell Dybala if City tomorrow offers him a salary that makes him earn 3x as much at Juve and he requests to leave. Because why would you keep a player who wants to leave? I mean he could $#@! the bed!
Why would Bayern have kept Ribery in 2009 and not bend over to Chelsea/Real, despite him wanting to leave? And after that tried everything in their power to change his mind? Why couldn't they be as submissive as we were when we gave them Vidal?
All those other and so immature top-clubs who keep their players and expect them to respect the contract they've signed. A tragedy.