It's not the norm, but only because Steam used its existing market momentum to make integration with its centralized servers the norm and like good little dogs, all other stores followed suit, rather than come up with something disruptive and pro-consumer like a reusable self-hosted servers solution for gaming.
As much as I love hating on Steam as anybody else, in this case it's not really true.
It's not the norm because the majority of peoples don't want to care about setting up servers or even choosing a server on which to play, they to simply start the game and either click on one of their friends in their friend list or click on "join game" and start playing. Anything more than and it's too much of an hassle.
It's the same thing with automatic updates, it's not because peoples were brain washed by Steam that they ignore the pleasures of manually downloading patches, it's because it is, for them, a lot more convenient to just not care about it and have it done automatically.
Concerning the subject of the thread, I have doubts, devs barely care enough to implement Gog multi-player and even then very few support the cross play with Steam feature; so I doubt most will be very motivated to implement Zoom MP on top of it.
The only way I could see it working is if it was a "in place
" replacement of the Steam API resulting in the devs not having to do any work to support it, otherwise you will be probably able to count the number of games supporting it on the fingers of a single hand. Alternatively maybe they plan on creating yet another Hamachi / Gameranger clone.