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

It's not that it's unoptimized, it's that running split screen requires 2x resources, resources the Series S does not have.

if you have X amount of work to do, you can't just "add optimization" and somehow you'll have less work to do.

if a game needs all the resources, then a well optimized game would still require all resources. but the unoptimized one would just not run properly.

optimized means "it uses the hardware efficiently". bg3 is a very well optimized game. it uses the hardware efficiently, and it uses all of the hardware. at a particular point, the only optimization left to do, is to do less work, i.e. to cut content.

optimization isn't some magic sauce you add to computer code to make it run faster. optimization is about writing good, performant code. at some point it's going to get as good as it can get.

the reason it needs higher specs than previous games is that it is doing a lot more than previous games. there is more work to do. what you're saying is akin to "this tiny car can do 100mph. why doesn't mine also do 100 mph when i stuff it full of bricks and give it a smaller engine?" well, it's because it has a lot more weight to carry, and less engine to do it with

He didn't say load a texture twice, he said twice the textures - which is a worst case scenario, but you could get if the players aren't near to each other.