Earlier quoted context omitted.
Its the manufacturer's (HTC, Samsung ..) fault that they cannot be updated.
And Google, because they have not implemented a default update system for the android core. Google only updates Apps.
If manufacturers / carriers change that to check for updates on their own servers, rather than google's - which they can do since Android is open-source, and so they all do - then that's how the system update mechanism will work.
I don't follow what you're suggesting google could do about that, apart from moving more and more OS functions out of the core OS and into google play services. Which is exactly what they've been doing.