Apple's revenue is 180 billion dollars. If they enter a new market, it has to be big. How many high margin billion dollar industries are there? The entire watch market is just around $50 billion dollars a year. If Apple wants to continue growing like it did in the past, the watch is a distraction. They need to aim for something much bigger.
Besides the financial reasons, I think it's the perfect time to break into the market. The switch to electric motors levels the playing field, giving newcomers an edge if they can move fast. But the most important part is the integration of all the systems in the car, most importantly the digital user interface. Todays cars have horrible interfaces. Manufacturers are too busy covering every possible niche with dozens of models, which makes it impossible for them to make a single great car. And even if they tried, they are too dependent on component suppliers to create a single, integrated experience of the kind Apple does.
There are two questions I wonder about:
1. How will Apple be able to make use of their experience in electronics when they move to a completely new industry?
2. What are the technical tricks Apple has up their sleeve? The iPod had hard drives and the click wheel, the iPhone had multitouch and a SoC that enabled a real browser, the watch had force sensitivity, haptic feedback, and a pulse meter. What will the car have?