Because there are other states in the world without those limits. Good example is the highway A24 in Brandenburg Germany. The accident numbers were too high so

Because there are other states in the world without those limits. Good example is the highway A24 in Brandenburg Germany. The accident numbers were too high so the local government was forced by law to impose a speed limit. The accident numbers dropped. So what did the local government? They repealed the speed limit because after all, the limit is only necessary for high accident rates. Guess what happened next? More accidents.