What’s stopping them? Laws in pretty much every state. Laws that have been one of Tesla’s biggest pains. They’ve managed to overturn some of them, but most the time they just get special exceptions carved out for them.

Regulation. Most states don't permit car sales without a dealership.

there are (good and proper) laws to prevent a car company from having a dealership network that is entirely dependent on them, and at the same time competing with that network by doing direct sales. since tesla doesn't have a dealership network they should technically not fall under those laws, but regulations tend to be 'no' by default (again, probably for good and proper reasons, since there are tons of incentives to game the system otherwise)

edit: as per this comment, seems i was wrong about this just being regulators saying no by default: https://news.ycombinator.com/item?id=18772642

Volvo is doing exactly that, it’s called Volvo Care if I’m not mistaken and includes some newer cars, service, maintenance, winter tires etc in a monthly fee.

Seems like Toyota Connected has similar projects in development - although maybe more focused for on-demand capabilities. Electric and self-driving capabilities aside - this sort of service subscription seems to have no major technological barriers; largely an operations + software problem. Seems like a good way to transition dealerships into service outlets towards larger scale fleet management.

In addition, Lincoln and Geely's Link and Co are also planning something like this.