The problem is that it makes more work, in the long run, for everyone.
Google is surely developing some features that "we" would like; but even
more surely, "we" will continue to develop features that Google would like
(for one, compatibility with evolving hardware). If they have to keep
porting their modifications to newer kernels, it also makes more work for
them. If instead they get their work into the mainline, then most of the
work would be done for them. That's a pretty big win-win situation.
Hopefully they'll figure that out someday.