Copying a shape by dragging it while holding down the Option key is a neat feature of Adobe Illustrator. It’s particularly useful with the Shift key.
Two problems. First, it is imprecise and slow. Cmd-C-V-rightarrow is more precise and quicker. Secondly, Opt-dragging is already used for moving either a handle while at the same time keeping the angle, or moving a node between handles, and other functions, depending on the selection. Opt-drag-copying might, in some circumstances, conflict with these functions.
I find Cmd-C-V-rightarrow a little cumbersome. Opt-rightarrow would be quicker.
the option key is used for so many other things, as mekkablue pointed out, so to add opt-copy would be quite dangerous.