Checking to see if something scrolls is way easier than looking at a design, calculating in your head if the margins look equidistant from one another thus deducing that it must be the bottom of the screen.
I always thought 'below the fold' was so overused or at least only for people who never use a computer, but I guess that's definitely wrong.
It's mostly because they want to use their jokes for a special down the road, but if someone records them during a smaller venue with the same jokes, it'll ruin the special.