Users need a new feature or a new power to justify transition. Learning of new OS is not free. Someone should reuse Android UI, but upgrade the OS to full Linux.
Mimicking the Android UI and UX is very trivial. The hard part is getting the OS to run on the mobile device in the first place. On top a tonne of custom drivers, it also requires way to either get accepted by the OEM locks or a way to bypass it entirely. This is getting harder by the day even with Android custom ROMs.