Dealerships were required because the carmakers started consolidating and dealing with incumbents in a consolidated market is terrible. But it doesn't actually work because you're still ultimately dealing with them indirectly. The actual solution should have been to break up the Big Three, though of course that happened anyway as foreign competitors came into the market and the argument for continued bans on direct sales has essentially evaporated.

The exception is service, because that's actually a separate market and not just a middle man on sales in the original market. Prohibiting manufacturers from operating service centers and leaving it to independent mechanics is essentially a ban on vertical integration and still makes a lot of sense. Because service for a particular model of car is in many ways back to being a consolidated market, since that model may need specific parts or tools and you still want to maintain a competitive market for service for customers with that model of car. You also want to make sure mechanics can service cars of multiple makes, since that fosters competition too.