From a business perspective, possibly because it's a huge market and Apple has a lot of cash to invest. It's quite different to what they currently do, but they are limited in how much they can expand in their current markets so they're probably going to want to try something new. Entering the phone market was pretty ambitious too (not as ambitious, but they were a much smaller company then), and they were prepared to take their time doing it. I believe precursor forms of the iPhone were in development for over 10 years before it was actually released.

I'm not going to say that Apple will decide to manufacture cars, or that they'd be successful at it if they did decide to. But I wouldn't rule them out either.