The time horizon for moonshots is way too long for the average VC. Google started working on autonomous cars c. 2010, and I'd be fairly astonished if they're available for sale to the general public by 2020.
Google is a public company, and I think the average Wall Street investor is even more 'short-term' than VCs. In fact, it should be a surprise that investors are not questioning such spending. What insulates these moonshot projects from investor ire is that as an expense, they are only a very small part of the total revenue of Google.
Page is shielded from such investors by a Board of Directors. Presumably they have a better understanding of Google's long-term vision. It's also worth keeping in mind that Google performs extremely well even viewed from short-term metrics.
I could see them partnering with Tesla or some other forward thinking automaker in the near future. It would be a very large investment to also build their own cars which probably wouldn't pay off as well as a partnership.
