System fonts have been an OS-level feature since the advent of OS with bundled GUI. Support of a larger part of the Unicode space in system fonts has been an OS-level feature since Unicode. New emoji has been a visible aspect of that, especially on mobile, for quite some time.
So it looks like it's not a "ploy", but an actual technical reason. Baffles me.
It really does seem to be something that drives adoption.
I rarely use those now, but I'm not immune to superficial hype. It's fun.
[1] https://developer.android.com/guide/topics/ui/look-and-feel/...
I don't know where they could keep them since I agree it would be a good idea to have something very similar to other oasis.
If the purpose of this was to bring them in line with other emoji sets, they could have just redone the few that show somewhat different expressions.