The type of 3D that requires you use glasses is relying on your stereoscopic vision, I.e. each of your eyes is seeing a slightly different image which helps your brain create a 3D perception of what you see.
The thing is, we only use stereoscopic vision at relatively short distances. At longer distances other effects such as relative size and parallax are much more important. Provided you don’t move, parallax isn’t a problem, so then it just becomes a matter of playing with relative size, shapes, placing objects in front of other objects etc, it’s all 2d techniques.