A few. But that doesn't have anything to do with the subject. People building their own rarely advances development in the automobile industry these days. Individual manufacturers with their vertical markets do most of the advancement.
We're comparing one market to another. It's easy to say you're comparing apples to apples if all the differences are irrelevant.
Computers are not automobiles. It's pretty well impossible to build a car so that nobody else can manufacture parts to fit it. You can design an OS to keep it from recognizing anyone else's hardware, and you can build a processor/motherboard that won't work with anyone else's OS.