Because you're focused on the visuals from a single user perspective...

1. There's the world state and game logic to consider as well, and this would be relevant even in a 2D sprite based game.
2. The article makes it clear that it's the couch co-op split screen that is causing the most headaches, with whatever additional overhead there is in maintaining another active character and rendering of the world on screen.

because split screen requires rendering stuff twice. and also needing to keep more stuff in memory simultaneously, depending on what two players might have in their field of view, instead of just one.

also, reducing the (subjective) quality by half, does not necessarily mean that you are now using half the resources. And also your game would look like shit compared to its contemporaries