Because millennia of religious conflict and friction between communities is not going to magically disappear even in the goal of a free and fair Palestine. It would be a process. You can’t just say “Everyone is free and fair now”, and expect everyone to shake hands and go about their day. People are still beholden to their own biases, that is not a risk that we should be willing to take. All it takes is one person or group to become “aspirational”, and you end up back at square one.

There will be opportunists, seditionists, religious extremists, and many others attempting to take advantage of the situation, in the wake of the creation of a Palestinian free state. That can be prevented and mitigated with a third party, if only for a time.

Think of it like a GDR scenario. The Soviet Union didn’t leave the GDR up to its own devices the second the war was over, because that would have been disastrous. They monitored and kept watch over the situation for several years until tensions had decreased and the GDR could be established. Jerusalem would be an extreme hotbed of hostility as everyone would try and make their move. That can be avoided.

Just because you say everyone is free and fair, doesn’t mean that that’ll automatically be the case. That is idealist, especially in the immediate aftermath of the destruction of the Israeli regime.