Driving through airport terminals is possibly one of the most chaotic possible environments, with people almost suicidally throwing themselves in front of you, not to even speak of the cabs, double lane parking, security guards telling you to move and other things. It's probably one of the harder environments to automate for outside of inclement weather.
Some airports have shuttles on private circuits to take you from one wing of an airport to another wing of the airport. The ones I've seen that have this either have a rail system, or a human driver. It would be much cheaper to just throw a bus with some software to track digital route markers and some visual matching for unexpected situations than to build a dedicated point to point system rail/track system (and maybe marginally cheaper than hiring someone, depending on other costs).
