Pressure and gravity

Answer:

This question addresses many different concepts in physics, ranging from microscopic to macroscopic interactions. Trying to understand physical interaction at multiple scales can be difficult.

Now lets think about all the other faces of the cube. There are still molecules colliding with each of the faces so there is still pressure every where. However, the density is the same at each of these faces, so each side has the same number of net collisions, but now there is no net force.