There's no Xbox App on Mac because not many people want it
There aren't that many requests for a Mac version of the Xbox App, which is the reason we don't have one.
The Xbox App, Microsoft's app that brings together Xbox One users and PC gamers with shared features like achievements, game DVR, cross-platform chat, and more, has yet to be made available on Mac. While most of these features aren't particularity exciting, the real killer is the lack of game streaming on Mac, something that can only be done through the Xbox App.
Game streaming allows users to play their Xbox One game on any PC running the Xbox App, which would be even more of a useful feature for most users running Mac, due to the usual lack of power in Apple laptops.
According to Xbox boss Phil Spencer, not many have asked for the feature. Spencer, who was answering a fan on Twitter, directed him to the relevant Xbox feedback page, which at this stage has 920 votes.