This isn't always the case, granted it happens.  A lot of times a ponzi fails because of math, and the site owner isn't running away with anything.  The whole idea is person 1 puts money in, person 2 puts money in and doubles person 1, now person 3 and 4 have to put in to cover person 2.  You can see how quickly this will fail, my example is people putting the same money in.