protect a dealership from competition by the manufacturer.

For most other products, the manufacturer is free to sell things directly from their website, and free to open its own showrooms or company-branded stores, but also sells through third parties. That mix works fine for the businesses involved, because the market creates the balances between them. Imagine if a company sold its products at low prices on its website, but didn't make it possible for third party stores to sell them at comparable prices: the result would be that fewer stores would stock or sell the product, and they might sell less overall, so the markets force them to learn a balance and not put dealers out of business if consumers value the dealers.