Apple will use it's dominant position to create lock in like how they did with iMessage instead of cooperating with other platforms on a common standard.
Oder friends and family are surprised when they want to video call over Facetime and find it hard to believe other people's phones don't have Apple apps.
The app allows iPhone users to create an event. Anybody on any device or browser can RSVP. The event can be shared as a link. Making an event invite app that only works for users on one platform would be pointless.
Also - non-Apple users have been able to join FaceTime calls via. A link for several years.
It's not the color itself that's the problem, it's that having one green user means the entire conversation falls back to SMS and thus photos, videos, etc are all degraded and you can't do more rich messaging things like reactions. This is changing with RCS but it is in Apple's interest to make it a social change rather than just a technological limitation.