They said they would make it an open standard. People took them on their word.
https://9to5mac.com/2018/06/06/make-facetime-an-open-standar...
Because theres a limit to how many physical devices they can sell and would like to keep growing after they hit that limit.
Its why Apple Music works on Android and has a Javascript SDK
It may not have too many takers (I’m willing to bet that most people currently using Apple Music on Android devices are iPhone users with Android tablets), but it would help families or groups of friends who have that one person on Android, and those folks would bear a visible monetary cost that they could partially recoup by switching.
Remember how surprising it was to everyone when Apple announced a windows version of iTunes in 2003. Jobs resisted the idea, and then publicly acquiesced, standing in front of a slide that said “Hell froze Over”. Aside from the additional iPod and iTMS revenue, the halo effect of widespread iPod exposure likely sold a lot more macs than platform exclusivity would have.