The resources required to develop and test kernel/base apps/windowing system are quite massive. Particularly the "test" part - the Linux developers and distros are completely dependent upon hundreds of thousands of end users to test our code on the millions of different hardware combinations out there.
The only other organisation which has the sheer resources to develop an OS for general PC hardware is microsoft plus their thousands of ISVs and IHVs.
I'll let you work out the rest :)
In other news, Chromium Blog announced that Google runs "more than 9100 individual tests, typically 30-40 times every weekday" for Google Chrome. Not to mention that millions of people test each version of Google Chrome on different software/hardware configurations.
{ Photo by Google. }