There are lots of reasons:

Naked corruption, be it financial or (more like since this is state level), nepotism.

When many of these laws were instituted, it was generally illegal for producers to own their own means of distribution. Movie studios couldn’t own movie theaters for example. That’s why streaming went from a small collection of collaborative entities with most things you’d want to watch, to four (or more) dozen, all price fixing and moving in unison just like the cell industry does.

Theoretically, tax money is more likely to remain in a state if a car dealership is local to that state. Ford selling vehicles in Georgia, for example, would almost surely send all their profits back to Michigan or whatever tax haven is cool these days (which wasn’t as much of a problem when these laws were made).

I’m not defending dealers, though. They are rent-seeking parasites that grossly underpay the people in the garage who keep things humming along. There is a very real dealership-owner (or children) to state politician pipeline in my state and I don’t think that’s a coincidence.