Android is already open sourced, It is Google play, Google Play services, and Pre-installed Google Apps that are proprietary.
Just like pretty much every other major opensource project - even Linux itself is majorly developed by a few corporations.
Why else would they keep moving things out of AOSP and into their proprietary frameworks?
Same for Linux itself - however in that case, there isn't incentive to make it proprietary, because the companies are either making money from hardware sales or from support contracts.