It's not like the Kimmel producers had to go out of their way to get these people. When the celebrity is a guest on the show, they ask them to participate in a segment they're making. After a few months, they edit all the videos into one skit. There's no clever writing or anything. It's "read this mean comment and look mad. Ohmygod that's brilliant."
Leno does the same thing with YouTube videos, except he pastes his head into the video. At least he used to do this. I haven't had the misfortune of watching a Tonight Show episode in years.