system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why does the U.S. have different state driver's licenses instead of everyone just having a United States drivers license.

If you move states and you have a drivers license already, you don't have to take a test again. All the laws are the same. So what's the point in not making it a universal license?
gpt
The Federal government only has authority over those things specifically granted it in the constitution. Everything else is up to the States to write laws to govern. Laws for driving vehicles is not under one of the things granted the Federal Government and cannot legitimately be argued to be covered by the things that are grated them. This means it is a State matter. 

> If you move states and you have a drivers license already, you don't have to take a test again. All the laws are the same. So what's the point in not making it a universal license?

This is not true. Some States do require you to take the test again when you move from another State, and not every State has the same laws. For example, some States allow you to turn right on red at stoplights.