When you're sitting in the harsh blue light of the American Idol thunderdome during the fourth-hour of the interminable finale, the cartoonish swoop of that theme music constantly ringing in your ears, your throat hoarse from yelling every time the cameras switch back from commercial, your tired eyes only able to see Ryan Seacrest's capped teeth—an eerily iridescent white flash in the distance, it's probably very difficult to think. Your senses are pretty much stretched to their breaking points, and your active mind is no doubt quivering inside your skull like a bowl of useless Jello. You're running purely on impulse—and whatever the stage managers tell you to do: Applaud! Scream! Get excited!
So when they direct your attention to a video of Jack Black, Ben Stiller, and Robert Downey Jr. dressed up like the Pips and singing back-up for a superimposed Gladys Knight, your reaction is based solely on instinct. You see Jack Black's face. You howl with laughter. You can't help it. Your American-Idol-finale-weakened brain thought: "That's Jack Black. He's funny. He must be doing something funny." and so you laughed. It was almost Pavlovian. Then Jack Black took off his pants, your stupid brain thought, "no pants = so so funny," and your stomach almost exploded you laughed so hard.
At least, that's the only explanation I can come up with for the raucous reception this pretty tedious video received during the American Idol.
Incidentally, this video is now R. Kelly's closing argument: "You believed that Gladys Knight was singing with the Tropic Thunder Pips, didn't you? The same people who made that video probably put R. Kelly in this sex tape."