I think this is cause most developers just end up using steam's APIs for these things and since steam will be most of their sales they don't bother. I have seen games using epic's solution in their steam distribution.

i do wonder how hard it would be to integrate multiple of these APIs so that the end user could invite anyone from any platform, though i imagine you'll still need some sort of middle man / lobby server...

Either way, I'm not a fan of having to generate lobby codes every time I want to play baldurs gate 3 with my friends who have it on steam