Except for when the Sixers know that the Celts don’t need the general consensus #1 player the same way they do. At some point, if they threaten to walk away, they leave the Celtics to either a) shop around the pick to someone else, in which case they don’t get the #3 pick back and likely won’t have the guy available that they want to take at #3, or b) forego the asset they’re gaining in exchange and just take who they want at #1. The Sixers correctly discerned that the Celtics value the additional asset gained more than the difference between the #1 and #3 picks, and it’s why the deal got done.
The depth of the draft plays in the Celtics’ favor in terms of allowing them to get an asset they view similarly, but it also hurts the value of selling the #1 pick, because everybody else knows the draft is deep too. They’re not going to pay an arm and a leg for the #1 pick, because they know there are other comparably valued players just behind him. It’s just indicative that the other GMs in the NBA are not as sold on the generational transcendence of Fultz as the media has talked themselves into.