Chrome on Android certainly has its own code paths but it's also got shared DNA with Chrome for windows/osx both of which support plugins.
So, I see this as a logistical decision more than a true technical barrier although that said, Firefox plugins on Android kind of suck: some kind of UI api policing makes them pretty awful.
What am I missing here? For all the ones I want and google doesn't, like adguard and archive.is there are ones which would be perfectly cromulent including ones authored by google.