That would be one hell of an ecosystem to bootstrap, even starting with a partnership with an existing company, even for Google. Building "a" car isn't that hard. Building a car that conforms with all regulations, has had all the testing done on it, can be manufactured at any useful scale, and, oh, also is something that people want to buy, is not a problem that should be underestimated.
(Yes, props to Tesla. They did it. Well... mostly, at least, but it's at least a reasonable guess they're on a success trajectory. It's still a hard problem.)
It would probably be easier for Google to manufacture their own computing hardware from scratch, and as far as I know, they don't do that. Yes, they get it customized because they are a big customer, and they've at least dabbled in phone hardware, but even that wasn't "from scratch". I've got a 2013 Nexus tablet, and it's got all sorts of things in it that aren't from Google.
As big as Google may be, they are not infinitely big, and they have the paradoxical problem of being too successful to take on every little business. Large profitable businesses can't economically afford to take on new businesses that are significantly less profitable than their current businesses, because the rational business decision then is to reallocate the resources being given to the unprofitable sub-business and give them to the profitable sub-business to make more profits. [1] The car industry is very hit-driven due to the need to sell enough cars to overcome massive fixed costs, but even at in their most profitable years I'm not sure the car companies came even close to Google's profitability. I'm not taking the time to look that claim up, because I'm sure that even if they did, they never did it year after year after year the way Google has.
[1] Obviously I'm simplifying a complex thing here. I understand that's a complicated topic, but details would be an unimportant-to-my-point waste of time for all.