It stems from franchise law which exists to give franchisees a modicum of protection from adverse practices by the much stronger party in the relationship.
I'm sure that's a part of it, but I'm of the opinion that car dealers and other really local business owners like them are the modern equivalent of the landed gentry.
Locally powerful people can have a lot of leverage, even against a much bigger national-level entity.
Car dealerships tend to be keystone businesses in towns; they wind up with outsized political power on the state and local level.