younger millennials grew up on multiplayer and online games, which were widespread and extremely normalized by the time we were old enough

I think that one part of that shift was VoIP. In the late 1990s and early 2000s, if you were playing, say, Quake or similar online, you communicated via text chat. Subsequent to that, a lot of games acquired VoIP support, which I think helped make communication in online games more accessible to a broader audience.

But another factor that I think affects playing multiplayer games for a number of people is having kids. Like, you're 18, you don't have that many immediate responsibilities, maybe. But if your kid's diaper needs to be changed or they produce some other emergency, getting an period where you can play realtime games with other people is maybe harder to get an uninterrupted time block for. Maybe slow turn-based games, like play-by-email type strategy games or something, stuff that doesn't have the same time constraints, would be more-viable.