Why are autonomous vehicles not used at airports for on-ground services?
Many of the ground vehicles need to have a trained human to make it useful for the plane (for example the fuel truck, mobile stairs, ...) so it makes sense for it to cart the trained human around. If you do that you may as well make said human drive. This means there is a high ratio of human controlled vehicles mixed with the autonomous vehicles. There are also the firetrucks that need priority, giving it to them sometimes needs human intuition in where you stop or move to to make a path.

You don't want the vehicle to path underneath an airplane and cause damage, or potentially end up in the jetblast.

Someone needs to monitor them and coordinate with the ground controller whenever they need to access/cross the taxiways.