Tesla also has to operate local showrooms and sales outlets throughout the world. In order to match a full scale manufacturer, they would need outlets in every medium-sized rural town in the US. It’s not an issue for Tesla now because they have outlets in big city tech hubs, but that won’t be sufficient unless they’re content staying a high end luxury brand. Dealerships run on lower and lower margins these days so there isn’t really that much to gain. It’s effectively the same reason that restaurant and fast food chains almost universally adopt a franchise model.