Bottles can't be outlawed (except in controlled areas like sports stadiums), and knives are mostly only a threat in close space (have you ever heard on the news of someone throwing one instead of stabbing/slashing).
Guns are a threat from a distance (or across a counter in a store or bank), and the threat only needs to be potential... like nuclear weapons. The threat is there IF they are used, so the possession is just as serious as using them.
There might be a lot more instances that aren't publicised or known to be replicas... as I'm sure that people being robbed at gun-point don't go to the trouble of verifying that the gun is a real weapon.
Prohibiting something doesn't mean it is a common threat... nor should a crime become a common threat before it should be outlawed, as victims of crime (no matter how few there are) would advocate a preventative measure if there is one that doesn't require much in the way of resources to implement.