Well pre-RCS they routed android users differently because they were not compatible. Google did something similar, but in reverse, adding back things like reactions, etc. to make android users not feel like they were getting a 2nd rate experience.

they don’t have to go into a specific app and hope that I’m looking at it

Do the others not ring your phone? I don't video call often, but when I do it's usually with Signal, and that definitely rings my phone.

You can do the exact same thing with any of hundreds of different messaging apps. The only advantage is that they're using the same messaging app, because it comes installed by default, you can't remove it, and they don't allow you to replace SMS with anything else. If you use an Android phone, it most likely comes with Google Messages pre-installed, which does the exact same thing.

In other words, it's nothing to do with "user experience" and everything to do with being in a particular ecosystem.

Literally all of that UX is the same and better in other apps though.

For example, every single part of your description applies to video and text conversations with my SO and friends, except we all use Signal. It "just works", and better than Facetime because it doesn't matter what device my SO and friends have.

With Facetime it doesn't "just work" at all with the large number of people I know who don't have Apple. That's a huge disadvantage which means that Facetime UX sucks.