Google is likely capable of creating its own operating system (if you don't already consider Chrome OS a legit operating system).
I don't think they are capable of such mistake ;-)
There are plenty of free more-than-good-enough OSs to build upon. Unless Google needs something more revolutionary than Plan-9, they don't need to create another OS.
The only reason I would find sane to develop an entirely new OS would be to use all these new shiny toys we got in specialized processors inside our computers. It would be awesome to have a machine with lots (hundreds?) of cores of varying capabilities that could be powered up or down according to load, with processes being transparently migrated between binary-compatible ones or between similar virtual machines hosted on diverse cores.
It's been a while since I have seen a cool new research OS.
Apple was failing. It had an ancient and crufty OS and needed to find a new OS outside the company because it failed to develop one internally at least twice. They could go with BeOS, NeXT or MkLinux (I run MkLinux on one Mac in my collection). NeXT came with Steve Jobs bundled for free.
At that time, Apple leveraged its bets making Java development a first-class citizen. It was not clear whether Objective-C would gain any foothold because it failed to get traction during its NeXT years.
Google can release something else, like high performance webservers, databases or other web infrastructure. They are in as much demand as probably an operating system. And they make good applications for a killer app.