That and round stones don't do as much damage as sharp ones, but I think in honesty it's more of a spectator sport like wrestling or MMA fights, people just want to see someone else get pulverized.
Kinda like when Ceasar fed people to the lion's after gladiator fights, it's purely entertainment.
Back then yes stoning was a way to punish criminals, you can only throw so many stones before your arms get tired but if it was a death sentence and you get tired after a hundred stones or something and he is still alive than yes someone else would have to take over.
But I'm guessing the majority of people their were purely spectators