Can't you play games on the apple tv?
Realistically, it would still cost about the same as an xbox/ps4, and the games would be pretty pathetic in comparison. Great games for a phone or tab, but not up to scratch for a console.
Console makers generally lose money on the actual console, but make up for it from games and accessories.
Since Apple charges a premium for hardware, you'd be looking at spending maybe ~10% less for an apple console than a PS4 that's capable of some pretty amazing games, bluray playback, and VR etc.
The fact the games are more expensive and will blow the cost out dramatically over the life of the console is of little interest to people buying today.
Lastly, since most people have both a phone and a tab capable of playing games on the app store, there's less reason to by a 3rd device capable of doing the same thing.