It is because it comes in conflict with other features.
But if you do F5, Enter NewName, Ctrl+P, Arrow down, then Return to start copy.. it works..
(Ctrl+P changes focus to FileOp Plugin box, and arrow down select the first that is Folder Content, that copy the folder content )
(Yes, it is two more keys and that is a pain, But can be change to just one extra key by assigning the the FileOpPlugin settings to quick load preset. then it is just an extra F-x key
like eg F5, Enter Name , F1, Return )