GM can’t match the lobbying power of local dealers in the state houses. Every state rep has a dealer or two in their district, and they employ staff who vote.
And, to be fair to the dealers, there is a long history of the makers trying to screw them over, and the public, too.
The dealership model is an anachronism from the days when the Big 3 US automakers were basically a cartel with a lock on the market, and predates the internet. For new sales it has no real advantages over what Tesla is doing, and for service the public would be better served by strong right-to-repair laws so people can have their vehicles serviced at any independent mechanic (or do it themselves).
And, to be fair to the dealers, there is a long history of the makers trying to screw them over, and the public, too.