A collision shape could literally be any shape possible. I mean, yes, cube shapes are common but not 100%. For example, for a character you probably want a capsule, not a cube or a sphere. Or if the mesh is not convex, maybe you need multiple shapes and a single shape could block an area the player should walk through.
That said, if you click the Mesh button on the top center, there is an option to create a collision shape from the mesh (for example a convex collision shape). I would say maybe they could add a box shape, that would be a minimally enclosing AABB or OBB, which would seem straight-forward for the engine to calculate. I've done this myself with a custom engine, it is fairly simple, and might be a good feature.