So you have to write a replacement for the Google apps and get driver support. Sounds a hell of a lot easier than writing your entire OS from scratch.
There were (even before Android) _multiple_ Linux-based operating systems designed for PDAs and/or phones (remember GPE? Qtopia? etc), and there are (even after Android) multiple non-Android Linux-based operating systems for phones.
Frankly, evidence seems to point that Google borrowed from the community much more than they gave back.