All of it?
They are (prohibitively?) expensive. (how much for the raw alumum sheet?)
They do not have a good method of attachment. (Dzus fasteners)
Prone to scratching. (Jack the wheel up, spin it and sand them out with 200 grit sandpaper)
How about it's hard to move the valve stem to the inside of the wheel and fill the tire from there? Or accommodating wheel weights?
Chuck the sheet ABS in a lathe and use a roller on each side similar to clay on a potter's wheel. Casting won't get the best result.
I got a set of 4 at a swap meet for $5.