For the same reason they don't make washing machines, or sell apples, or build houses - they believe their money is better spent on other things. Apple think they'll make more money in the long term innovating on consumer electronics.

That said, I wouldn't be at all surprised to see an Apple car within 5 years of the first autonomous vehicles going on public sale.

I don't follow the question. Why doesn't Walmart?

Because Apple is smarter than that... they will do what they do best...

Let someone work out the kinks, then step in and make it better...

Why waste all the money on all of this regulatory squabbling... let Uber and Lyft do that, give all the free rides, lose money to make this thing legal... then when it's legal, swoop in and take over...

Why does Google want it? It's simple. Google is an advertising company. The more free time you have where you have nothing but your phone or computer, the more time you'll spend searching for things, and therefore clicking on their ads. An average person with a 30+ minute commute would sure as hell do a lot of shopping if they didn't have to drive. While we are all waiting on self-driving cars, this is another step in that direction. They want to validate the idea that having people drive less means more profit for them.

Apple is not an advertising company, they are a consumer electronics company. You won't be buying more iPhones by driving less, so they don't benefit from self-driving cars or car pooling quite as much.