Because they ride in them, naturally.... Limos typically have V-10 or V-12 engines. Bigger than most sport cars.
Don't expect those will ever be banned... except perhaps for you and I to have for weddings and proms. They could still pass laws against that, of course.
Or one final occasion. Little Richard said it best back in the 50s when he said the only way a black man could ride in a cadillac was at his funeral.