1. A national dealership network is at minimum, a soft-guarantee that your car can be maintained/recalled/fixed without you jumping through hoops.
Unfortunately, that soft-guarantee is saddled with all the other baggage of car dealerships. Pushy sales-people, extended warranties, predatory loans, etc.
2. It forces you to buy local, so not all the money flows out of your community, and into Detroit - some of it stays to circulate in your town.
3. People want to sell/trade their cars in, and Craigslist can be a pain in the ass.
4. People like to test-drive cars. People really like to test-drive used cars. See point #3. By the time Tesla starts holding used car inventories[1], and lets local buyers test-drive them[2], their 'showroom' starts looking less like an innovation, and more like a non-franchised dealership, where instead of the franchise owner skimming profit off the top, it just goes up the food chain, directly to corporate.
[1] Which they are.
[2] Which they do.