When I was a kid playing Quake 3 I had different servers bookmarked, knew the players that ran the servers and the general crowd that hung around. Had friendships and rivalries.
Modern multiplayer you may as well be playing bots. There's no community.
They're just all single player focused.
It's not the only thing going anymore, but also people have orders of magnitude more choices. That's the thing that's missing today. Nothing is "dead", there's just too much competition now.
Honestly the aforementioned games have had much more staying power than more recent multiplayer games. And they're mostly played by old men like me.