Cognitive dissonance. The inability to learn new ways of doing things.
no need for name calling people, most do have valid reason but many can not communicate or express it for everyone to understand in single post.
I think people have their good reasons, for example : reading PDF side by side related to task in hand, not all website shows well in full screen or on ultrawide monitor, copy and pasting side by side vs switching screen.
the problem with "optimized" for full screen, is enforcement and reducing way people can achieve task efficiently, instead of learning how people use things and help them achieve it easier and faster.
again with no GUI lag, these things are easier to perform and not less ideal in anyway.