2D Universe

[Nafensoriel]

Again... not quite.

Learned behaviour and how the brain processes information is not the same thing. If a human grew up in low gravity their actions and their interpretation of their surroundings would differ drastically from land bound humans.
Humans do not process the world with the consideration of gravity as a thing. We LEARN gravity is a thing and start to base how we react off that constant during our formatives years. This is well known and fundamental in how we learn to walk. We arenot born with an innate understanding of gravity as a downforce.

Additionally in every possible case humans, and again all known life on earth, will process more than one sense in a 3D way. In no shape or form does any known life on planet earth use exclusively 2D understanding. Even your case of using "gravity" is a second sense. To orient with gravity you have to automatically accept the "down" pull to take in the "2D" expression of your eyes and your brain calculates everything this way. It's why you don't need to be taught how to throw a rock and be decently accurate. You are calculating in 3D. There really is no argument whatsoever to life seeing in 2D. There is an absolute mountain of evidence and papers about how humans can only see in 3D.

You need to differentiate between the process and the learned behaviour. It's just really bad science otherwise.
The reason we orient video games to 2D is that we control them in 2D. Ever work with CAD or prototyping software? There is a reason their VR is advancing considerably faster than video games. It's frustrating as hell to know how to move the object in front of you but being limited by an obtuse control scheme with a single mouse based off x/y.

TLDR: Want to know why games don't have more "z-axis" stuff? Look at your mouse. It's not PEBCAK. It's your tools.