Because their systems specifically don't work with each other. It's a stupid design, whether they consciously made that decision, or just didn't think about it.
I'm not sure if you're joking or not. They already have the technology for phasing. Hell, they even have the technology for groups to sync into the same phase. How fucking hard is it to just have the mentor be put into the same phase as mentoree?
Not sure if you're stupid, or just not understanding what I'm saying. What I'm suggesting doesn't change their playstyle or preference for phasing whatsoever. It would just let me play with my wife using a system they've used as a selling point for the game.
I don't dislike phasing. I dislike that my wife,when scaling down in level specifically to play with me, is unable to do so because their systems conflict with each other. The entire point of the mentoring system is so higher level players can scale down to play with you without ruining the experience (by 1-shotting everything) or make it pointless (offering no experience because you're grouped with someone too high). However the mentor system is completely fucking pointless when she can't join my phase because she's higher level and already did that quest area.
Except WoW hasn't ever advertised a system specifically for friends who are further progressed than you to scale down to play with you. So why copy a design of a system that conflicts with your own indigenous systems?