Well, the center bolt also goes through the metal bracket, which is bolted to the transmission. The center bolt shouldn't move I would think. Also, it has the weight of the motor resting on it, so that is also preventing it from moving.

If you are trying to remove it with a ratcheting socket wrench, it is going to be tough I would think. I would suggest using an impact gun. Also, I would get a jack under the motor in order to help relieve the weight off the mounts for the bolt to come out easier if needed. If you jack up the motor, be sure to use a piece of wood between the jack and the oil pan to evenly distribute the weight so the pan isn't damaged. I have a picture floating around of me using a Haynes manual for this, but I can't find it for some reason. haha

EDIT - Found it