This is because the PS! Ladder uses a rating system called the Elo rating system. You may heard of this before, because it was originally made for rating chess players. Nowadays video games use it too (like League Of Legends).
One of the striking features of Elo is that it can start either from 0 or 1000, both would mean nearly the same thing. Most FIDE tournaments for chess however start off with rating 1000 though, so it's kind of a set standard.
Also, when you keep a starting value that high, it's easier to give more specific points, and pin point out a better player by margins. On the other hand, starting with 0 means that points may have to be rounded off when in decimals, making it a little less specific. Major league games need absolute best players however, so they always start out with ratings like 1000, 1200 and 1500. Some even 100. The only problem with that is inflation/deflation in average, which can be remedied with provisional rankings and such, but I don't think PS! uses it.
For more info on Elo ratings.
Hope I helped!