They can outsource silicon development. Should not be a problem with their money.
In comparison to dotcom development teams, semi engineering teams are super cheap. In Taiwan, a good microelectronics PhD starting salary is USD $50k-60k...
Experienced teams who have designed high performance microarchitectures aren't common, because there just isn't that much of that work done.
And when you're eventually going to spend $$$$ on the entire process, even a 1% optimization on the front end (or more importantly, a reduction of failure risk from experience!) is invaluable.
At the scale that Google is at, it really wouldn't surprise me if they were working on their own silicon to solve the problems that exist at that scale.
"""Regularly upgrading network fabrics with the latest generation of commodity switch silicon allows us to deliver exponential growth in bandwidth capacity in a cost-effective manner."""
https://conferences.sigcomm.org/sigcomm/2015/pdf/papers/p183...
The next step in that set of goals for their data center would be custom silicon where merchant silicon doesn't provide the right combination.