The odds that rock beats scissors are different from the odds that scissors beats paper; odds increase with luck of the draw as well as skill; and rock can in fact beat paper, it's just not good at it.
Also, there are different kinds of rock, paper, and scissors with their own sets of odds, as well as the occasional rockpaper or scissorrock, and sometimes there's an ace of spades that people will swear is obviously a rock. Or maybe paper. Definitely not scissors, though.
And at times you'll get scissors which cut rock as well as paper (and other unwary scissors). Then everybody plays Tic-Tac-Toe for a little while until the scissors are reminded of their place in the precarious balance of things.