Online multiplayer is a gaming staple, but not every big title lets people play together at home. Games like 'Baldur's Gate 3' and 'Diablo 4' are reinvigorating the genre.
Forget split-screen, a lot of people have two monitors these days. Steam hardware survey has "Multi-Monitor Desktop Resolution" as "other" for 50% of their users, which I'm guessing means about 50% of people have multiple screens. Think about it, game devs.
The issue isn't screen real estate, but processing power
When you do split screen, you're basically having to render two games at once (a bunch of stuff can be "shared" like physics and such, but you still have to render two PoVs at the very least). This is helped slightly in split screen by the fact that you're rendering a much smaller PoV for each player, with multi-monitor split screen, you lose that edge.
Basically it could totally be done, but only on pretty decent hardware and/or a really efficient game