Because even if the assigner does pre-assign rooms, someone who checks in early could make a request at the desk, and because you are not there yet, they will bump you off of that request, and grant the one that has arrived.
Also, folks can extend their stay unexpectedly and still be in the room that you were assigned, so then that messes everything up.
Believe it or not, even if the resort is booked solid, if a guest insists, they can extend their stay indefinitely as long as they keep paying for it.
Obviously the resort can tell them that they are booked solid and ask them to move elsewhere, but the law states they have to let them stay if the guest insists.
Carol