I'm pretty sure that EA attorneys and the auto insurance companies cautioned against allowing cars on the roads. Roads are, as we all know, meant for elephants. That's why they (the roads) are grey ... so that the elephant doesnt forget where he is supposed to walk. Therefore, there is too much risk that a car would collide with Stampy, and medical costs when the driver is squished by an elephant would be enormous.
On the other hand, with so many Springfielders owning flamethrowers and rifles, one would think that The Elephant Problem could be easily contained. It would make for an awesome joint/gang task. Stampy, versus Brandine, Hank Scorpio, McBain (and a host of others). All on the road. And Lurleen could run the barbecue (24 hour task)