Why isn't Canonical using Android?

Now that Canonical is talking to OEMs and supporting other devices there's talk of Ubuntu for phones, but wouldn't it be better to create their own Android fork and keep compatibility with that OS' apps?

One of the first problems with a new OS is the ecosystem (see webOS) and Android already has more apps than you can shake a stick at, why not use that instead of trying to port Ubuntu apps to ARM?

Amazon has been very successful by forking Android for the Fire, why not do the same?