Rendering more screens is more demand on the graphics card.
Also, not everyone has multiple screens or their screens laid out in a similar arrangement and/or the same resolutions.
Beyond these though, there shouldn’t be that much limitation to the idea.
I currently play Baldurs Gate 3 couch co-op split between two screens. There doesn’t seem to be a way to do it natively, since it recognizes each monitor individually, but doesn’t see their connections to each other. I have to do a borderless window and then drag one side of it to the other monitor.
Anyway, I wish there were more built-in options to spread a game across multiple screens. Years ago I used to play Rocket League across three monitors so I could see more of the field while playing.