Can you do this? "You see the silhouette of a cube, viewed from the corner. What does it look like?"
Or mark the edges of a cube into thirds, and cut off each of its corners back to the marks. What does the result look like? Can you mentally rotate this shape around?
I don't really see anything. If I attempt to visualize a cube from the corner, I can maybe say I can see the corner and three edges leading from it (to be honest, it's hard to say that I truly see it, but maybe a vague outline of three lines). I don't see the remainder of the cube.
If I try to imagine a cube, and then mark the edges into thirds, I cannot imagine what remains when I cut off corners back to the mark. And the cube itself is more of an idea of a cube rather than visual representation.
I cannot mentally rotate the shape as I do not see a shape.
I'm sure it's hard to believe, but trust me, it's hard for me to believe people can visualize in the way people describe.
Or mark the edges of a cube into thirds, and cut off each of its corners back to the marks. What does the result look like? Can you mentally rotate this shape around?