I haven't heard of any plans to change anything. For an answer as to why they don't have it: a lot of games don't because they use the built-in networking APIs for these services.  If you implement Valve's or MS API as they come, there is no crossplay.  Any game that does feature crossplay planned it from the beginning and have their own complete and independent networking infrastructure that does NOT rely on these APIs. I am about 95% sure that is a sufficient explanation with no gotchas that I missed :) 

Even if you have your own newtork infrastructure I think you can still use the APIs to implement vendor specific online service functionality like friend invites; so it might seem like these games are using the network services, but it's just built on top of their custom implementation.

Also, no reason to swear about a feature they never advertised having. Cross-play is not your god-given right.