It's the network effect. I suffer from this too. If only Apple would adopt RCS but they won't because they like their walled garden.
It sounded like it got to the point where phone companies were delaying so long (at least in the US) that Google is basically forcing their hand by doing everything themselves if the carriers don’t.
Meanwhile iMessage has been running for 10 years.
That said I agree with you that I see basically no reason for Apple to add it. That’s up there with iMessage for Android in probability.
However, I doubt people on other platforms would really appreciate it, because the level of integration won't be there.