A shape is like, well, you know, it's like a thing that has edges and corners and stuff. A silhouette is more like, it's a thing that's been punched into the wall of reality with a fist full of angst.
Shapes get bored with silhouettes because they're like, "Ugh, I'm a circle, a square, a triangle, I've done it all. I'm a shape of a certain dimension, with certain edges, I'm so done with silhouettes."
Only if the circle is made of cheese. Or if it's been possessed by a demon. Or if the demon is wearing a fedora and a trench coat. That's when you can fit, kid.