We're bidding whatever his value drops by this season to have him playing for us this season.
At the moment, that's more than Everton are willing or able to afford, and Everton aren't a big enough draw for him to force his way there (and earn the associated reputation that comes with pulling a move like that).
It's gambling on a known very high quality young player being willing to put the effort in (even if that's just to ensure he gets the move he wants) and improve his consistency, against gambling on being able to bring in a brand new player (or players) and then those players being able to fit in and give us as much or more on the pitch.