Because your premise is flawed.
Replacing an ill-suited player who has no practical function within this system with a player, scouted continually, who is a correct profile is absolutely the right choice.
And that's before considerations of "pure profit" and what Garnacho leaving for £59m does for our spending capital moving forward.
Even his so-called potential is debatable. The egregious error is presuming that a player being this good at 20 means that he will be that much better when he's 25., when there's plentiful evidence that quick and early growths can, and do, top out very early in their careers.
Garnacho isn't going to add qualities to his game that make him viable for no. 10 role. He doesn't have that in him. Not the passing, or the vision, or the decision making.