Question

why are computers so good at 3d graphics

Answer 1

Actually all gfx you see on computer is not 3D, it's 2D plot of 3D objects calculated thru the usage of simple yet complex Math and Physics functions optimized for computer to calculate.

There are main two parts,
1. Graphics Engine - Does all painting work in screen
2. Physics Engine - Do all physics as you studied in Classical Mechanics
Physics Engine makes a virtual world in computer memory then the Graphics Engine does plot the front scene of camera which looks 3D ( http://en.wikipedia.org/wiki/Perspective_projection#Perspective_projection) to you and this goes con continuously with effects which doesn't make you fee the thing you on screen is just 2D image.
This thing is called http://en.wikipedia.org/wiki/3D_rendering

I'm not expert but this is what i've observed till now.

Answer 2

3D exists, I think you are talking about 2D in a 3D space... Sometimes things are 3D, sometimes 2D calculated to fit into a 3D realm.

Answer 3

your question is not clear....