I can't really see self-driving cars being any more than a novelty. I love driving and am sure the majority of the population would agree with me. Why don't they make "smart" cars first, ones that automatically communicate with other cars, automatically reports accidents or poor driving conditions, etc. I know "there's an app for that", but the vast majority of drivers don't use them. They should come "baked in".
That, plus Apple can't even manage a flawless iPhone/iOS launch, let alone develop an automatically driving car.
First off, I agree that it would be very hard to just up and build an automotive company. It's about as likely as Exxon would make a good smartphone. Sure they have the money, like Apple, but what do they know about it? I don't care if Apple hires 10,000 people, if they aren't named Elon Musk, they don't know how to build a car company--his achievement is crazy, and look how few he's able to build and how long it's taken.
After you bother to design the thing, you need to figure out the enormous supply/resources necessary to build said car. Then you have to build the factory (which you've designed), then you have to build up a transportation network that can ship big things like cars, build your own network of retailers, and then supply them with parts, too.
No, they aren't likely to be so stupid as to actually try to build a car.
But, self-driving is coming, and maybe Apple feels like they need to be in that game. I think you will be letting your car drive you around soon, but it will start out as optional on cars (like auto parallel parking). Google has thought this stuff out already if you believe Eric Schmidt, and they've been working with the auto makers.
Giving up our own cars for AI taxi is the future, and that's still probably 20 years away.