That's normal. What you see on AI are not "real" GSX vehicles, they are default Ground vehicles that have their models replaced by GSX so, they "look" like GSX, but are not controlled by it.
The simulator has an hard limit on the maximum number of objects which cannot be surpassed, no matter how much powerful your system is (so we pretend to ignore the possible effect on fps) so, if GSX tried to do what you are suggesting, it will never work, because the number of objects will be too high for the simulator to handle, resulting in objects disappearing and possibly the whole airport as well, which is what already happens when too many AI are in view on large airports.
