Suffice to say, if it was as easy as waving the magic wand you think it is, it would have happened already.
Instead, if you tried to wave it, handset makers would have walked and just done something else.
Heck, some did!
Besides, why stop at Google?
Why not blame the carriers for devices like this be on their networks?
By your logic, if Verizon told Samsung tomorrow said "you must update your phone monthly with security updates in order to stay connected to our network" it would have just happened!
Maybe instead of assuming a vast lack of caring or outright maliciousnss, you should consider that maybe it's not the trivial thing you are making it out to be, and that's the real reason it hasn't happened yet.
Yes some have and some more would have. And then it would have been up to these holdouts to come up with a viable OS. Good luck with that. Almost everybody who tried has failed.
There is no doubt in my mind that Google could force it successfully, but why would they? Everything is moving in their direction anyway and users don't seem fazed.
Surely Google is not intentionally refraining from solving the high fragmentation it is suffering from (which makes developing for Android harder, and keeps some people from receiving security updates)
Because it causes friction, it's a distraction for management, and there is a danger that some of those rebel handset makers could have some success or get poached by Microsoft/Bing even if they fail at creating an alternative OS.
There is risk without much upside for Google compared to the gradual approach they are taking now.
Walking away from Android is not currently a realistic option for most of them (recent Sony Sailfish news non-withstanding).
If only they had thought of this. Oh, that's right, I forgot, it's not that this is much more nuanced and complex ecosystem interplay, it's that it's very black and white and they uniformly don't give a crap.
Sadly this also leaves us with the current sorry state of affairs. I don't think much "could've" and "would've" would be helping there, but I would very much like to see a major economic force guaranteeing some common grounds for devices. Imagine what an "IBM clone era" of phones could mean - all it would take would be a common ground w.r.t booting and a couple of drivers.
Sometimes I'm really left wondering why we can't have such nice things...