Split screen open world games effectively have to be able to run two copies of the game at the same time. This isn’t a traditional split screen coop where two players are always within one “level” together, and thus all the game code can run just once for that “level”. All the physics, ai, memory, textures, all thr subsystems are running just once.
If two people can be on oppsite ends of a world, that’s two totally different sets of physics, ai, memory, textures. Everything has to happen, twice.
Basically, believe the smart person who made the game instead of dreaming up reasons you think they are wrong when they literally made the game and told you the problem.