One of the causes was just mentioned.

We see everything with our right or our left eye from different points of view.

But if now we close one eye and look at the stage with the right eye only, the plastic effect does not disappear.

The psychological causes for this perception of depth with one eye are essentially the differences of apparent size, the perspective relations, the shadows, and the actions performed in the space.

Now all these factors which help us to grasp the furniture on the stage as solid and substantial play their role no less in the room which is projected on the screen.
We are too readily inclined to imagine that our eye can directly grasp the different distances in our surroundings.