That’s up to developers. Shadow Warrior 2 is available both on Steam and GOG. Both versions have playable online co-op.
Publishers probably have a final say in this, since Gearbox published games are on GOG. (And 2k’s, so I don’t know what’s up with BL not being there. Maybe they prefer using Steamworks for online play.)
Also, GOG Connect:
You’ll be able to permanently import all the [Steam] games listed below to your GOG library – assuming you own them. The eligible games are limited-time offers made possible by participating developers and publishers, so stay tuned for more games to come.
It’s up to developers and publishers to reach out to Origin, Uplay, etc. to sell games on their stores or even link accounts and give free copies of already bought titles.
I have a feeling they tried, but said stores did not want this. Clearly it works for GOG.
As for Epic, they could have put their fortnite money to making a better client and store.
Sell games at $50 and cover the $10 out of their pocket while keeping their revenue share AND allowing other stores to sell games at regular $60.
They don’t even need to drive customers to their store and build a userbase, because they already have one with fortnite.
They chose the scummy way. I can only hope it comes to bite them later.