Unfortunately, the hard part of an operating system isn't in a cool API and a rendering demo. It's in integrating the fickle whims of myriad hardware devices with amazingly high expectations of reliability and performance consistency under diverse workloads. People don't like dropped frames when they plug in USB :) Writing device drivers for demanding hardware is much harder than saving registers and switching process context. The Linux kernel has an incredible agglomeration of years of effort and experience behind it - and the social ability to scale to support diverse contributors with different agendas.
Microsoft, with its dominant position on the desktop, famously changed the 'preferred' APIs for UI development on a regular cadence. Only Microsoft applications kept up and looked up to date. Now Google has such a commanding share of the phone market - Android is over 80% and growing http://www.idc.com/promo/smartphone-market-share/os - they have a huge temptation to follow suit. Each time that Microsoft introduced a new technology (e.g. https://en.wikipedia.org/wiki/Windows_Presentation_Foundatio... WPF) they had to skirt a fine line between making it simple and making sure that it would be hard for competitors to produce emulation layers for. Otherwise, you could run those apps on your Mac :)
There are many things to improve (and simplify) in the Android APIs. It would be delightful to add first class support for C++ and Python, etc. A project this large will be a monster to ship so hopefully we'll soon (a few years) see the main bits integrated into more mainstream platforms like Android/Linux - hopefully without too much ecosystem churn