I think people tend to underestimate how much damage to a fragile human body even a low speed accident can cause. Mainly because consumer vehicles have an incredible number of safety features like seat belts, air bags, crumple zones, reinforced passenger compartments, etc. to keep us safe.
I have no familiarity with how specialized airport ground vehicles are designed but seeing as how so many of them are completely open air, or don't have doors, I wouldn't be surprised if they're not required to meet consumer safety regulations. Probably precisely because they're supposed to be operated at lower speed by trained operators.
I guess my main point is don't underestimate the danger of even a 15mph collision into a stationary object. If not for modern safety engineering, that's easily enough to kill you. Heck bicyclists not wearing helmets die at slower speeds than that.