Well we need to break some thing down here, there are plenty of real solid pranks on Youtube, but the percentage of those to all the other garbage is 0.01%
Everyone else just worked out the trick to get children excited and coming back for more, you name your video "X PRANK GONE SEXUAL!!!", or "X PRANK GONE WRONG!!!", put some boobs or butts as the image and you get yourself 1-10 million views. That is instant rent money for people who would normally spend their entire life behind a fast food counter. Yes those videos are horrid, probably the only thing that goes lower then pewdie but so long as it makes money someone will do it.