Not everything from Android is in the mainline kernels. There are still a few bits that are Android specific that would need to be reworked to even be accepted into staging.

There has been a lot of great ARM work lately in mainline too but there are still quite a few drivers missing that you can't even get docs for (even with signing NDAs) so the drivers for mainline get written based on the BSP-ish kernel drivers which aren't always written the best.