They are 3D rendered, however they are designed and meant to be experienced on a 2D plane.Except they are not 2d games. lol
This is like trying to explain 3D to a being that can only experience the world in 2D, you're not getting it.
In VR you're in the 3D plane, you control and experience within the 3D plane. This method of control and experiences is not possible while experiencing it outside of it.