The Nexus lines are not the only Android devices out there. Google has a huge interest in the rest of the Android lines and wouldn't want to piss off OEMs by in

The Nexus lines are not the only Android devices out there. Google has a huge interest in the rest of the Android lines and wouldn't want to piss off OEMs by including an app that would void their warranties. I definitely agree with what that one Reddit comment says - if you can't sideload it then you shouldn't flash your ROM.