I don't know if you noticed or not, but the GPL licensing terms only kick in
during distribution. Seeing how a corporation is a independent legal person I
don't think that moving software and hardware around internally really counts
as distribution. And I don't think that Google has any plans on selling its
systems to other people.
So the GPL is pretty irrelevant.
So it is just a business case of whether working with the kernel community is
going to be more profitable or not. And so far they decided that taking care
of stuff internally is a better approach. Maybe that will change.