When viewing this stunt through the efforts of another subculture besides skateboarding—BMXing, Scootering, freerunning—something interesting happens. The bias of being a skateboarder no longer clouds the vision, and it becomes clear how asinine attempting this is.
I've been thinking about this for the better part of a day now. I could barely stand to watch the embedded clip of the BMXer and purposefully didn't click on the links to the other videos, but I think I've seen Boulala's tries (and Sorry as a whole, for that matter) about a hundred times over the past 20 years. All instances show people trying to push the boundaries of what's considered possible in their respective endeavor, but I actively avoid watching unless it's someone trying to push the boundaries of the endeavor I consider myself a part of. Not only that, I also have a hard time understanding why anybody would jump down a 25 stair set WITHOUT A SKATEBOARD. Taking a step back, I realize how ridiculous this thinking actually is, yet it's exactly how I feel. Cognitive dissonance is a powerful thing.
Anyway, the answer to why anyone would try to jump it is probably, as Mallory said, because it's there.