Because we haven’t got unlimited funds. Passing up a big transfer fee and continuing to pay Mo the equivalent of three other players’ weekly wage is a cost we arguably can’t afford to pay when the squad needs investment to improve on this season’s outcome.
No, we don't have unlimited funds but nor are we any longer in a position where we have to sell to buy. We've let some big names go for free in recent years - Firmino, Gini were both first-team regulars at the time they left. Then there's Keita, Ox, Milner, Div, Lallana, Clyne... And not getting a fee for any of those didn't stop us buying Nunez for a hefty fee or bidding £100m+ for Caicedo.
We're never going to compete financially with the cheats, but that's because they're cheats.
The only thing we'll win by selling Mo is the net spend league. And if that's the height of your ambitions, you're looking at the game the wrong way.