Car sales are not registered with the US government. They’re registered with the state government. The two do not share physical infrastructure, data, policies, or even common goals, unless a specific agreement has been worked out between an individual state and the feds.
To me this feels a little like saying "the federal government doesn't know when people are born because births are registered with local governments". In practice this is all a matter of state capacity to keep track of this information. Given political will to make it happen, I don't see a reason why information about car sales couldn't make its way to the federal government in order to make tax filing simpler.
