Direct sales were forbidden as a consumer protection back when dealerships were disconnected franchises with no licensing or direct relationship with the car manufacturers. To eliminate unscrupulous dealerships but protect the ones that were doing useful services the regulations made a compromise that eliminated direct sales.

It may be aggressive lobbying that keeps this status quo long past the original intent, but the original compromise came from a good place (eliminating "lemon dealers" and "lemon manufacturers" that refused to provide service/support/recalls).