Yeah, I couldn’t find a direct link to any blizzard statement either but with so many articles on it, it’s hard to find the original these days.
Best I can find is a eurogamer interview
"So I was looking on the Diablo Subreddit to see people’s general reactions, because that’s a good gathering place for them, and one note I saw mentioned couch co-op and how it was a console-only feature. Firstly, is that right?
Rod Fergusson: That’s right.
And this person then wrote an impassioned post about couch co-op and the salient point seemed to be that why was a series that originated on PC now lacking an additional feature that consoles get?
Rod Fergusson: It’s a technology question. Trying to do shared-screen co-op on PC is much more challenging when it comes to account management and how you play together.
Why - how is it different from console?
Rod Fergusson: Well, console is pre-set up for that in terms of the ways you can associate the accounts to it.
Oh I see.
Rod Fergusson: And it’s been that way… Having split-screen or shared-screen co-op on consoles [has existed] for a long time.
Because those systems allow multiple users to be signed in?
Rod Fergusson: Right, exactly. Whereas trying to get two Battle.net accounts signed into the same PC at the same time… It’s a technology problem.
And the notion of two people sitting together at a desk: when you prioritise the problems you have to solve, solving for two people sitting at a desk, playing on the same PC, is lower priority when the majority of couch co-op that’s going to happen is going to be in front of a sixty-five-inch TV.
Is it a solvable issue?
Rod Fergusson: Um, not sure. I’d have to talk to our TD [technical director]. I don’t know if it’s a game-solvable issue. It might be a platform-solvable issue but I didn’t know that it’s a game-solvable issue, like [with] support from other aspects like Battle.net. I think there’s more tech that has to happen."