Somehow it's only iMessage which doesn't have an android or desktop or web app, despite Apple having more money than every other messenger app I mentioned.
> your family members chose Apple when they purchased their phones.
Apple chooses the default and integrates it into the OS more deeply than any third-party app can be integrated. It's not a free choice... and then Apple also refuses to provide open access to this ecosystem to other devices.
I know other people have sometimes said that it's an anti-spam measure to tie the iMessage account to an apple ID which is associated with a purchase. I'd be fine making an apple ID and paying up to $300 to get iMessage access for it if that would allow me to not use iOS and still communicate with my family (via an officially supported / recognized android + linux iMessage app).
When my iPhone finally breaks (and may it be soon), I am planning to get a mac mini server and install https://bluebubbles.app/ to solve this.
I am mildly worried that apple will eventually ban me for that, as they did with beeper (https://news.ycombinator.com/item?id=39156308), and also not thrilled about the increased electric bill that'd entail.