It makes no sense, and how are we meant to create the shape of what we want.
The reason why ROBLOX disabled the custom meshes is because some people decided to make some inappropriate meshes. The closest thing to custom meshes is the CSG implementation. You can use that to somewhat mold what you want.
Weirdness. They keep on making their own content rather than letting us make our own meshes for our own needs, so we still mangle old roblox ones into whatever we can.