Plus.. stuck car means that another, if spun on same place (and if there's some reason like oil or more water on that place chances are bigger) could collide with it, or, probably even worse, hit marshal or driver who's exiting car. Jules Bianchi tragic crash in Suzuka is case that proves the point.
If there's tarmac instead of gravel, cars are removed much faster and then there's much less chances of hat kind of dramatic crashes.
edit:
But, on the other hand, Moto races are something completely different, gravel is their best solution as vehicle stopper, as we may see from Salom's death.
So, guess it depends what kind of racing you want and, clearly, same tracks shouldn't be used for both. Many, sadly, do.