The NCAA tournament factoid of the week, first calculated by Ken Pomeroy and since repeated by Grant Wahl and others, is that the odds of a Final Four constisting of all number one seeds is only 3.5%, less than the odds of a sixteen upseting a one. Ha! Your brackets are screwed!
Here's he thing though. That 3.5% outcome is still more likely than whatever four teams you happened to pick. By a lot. There's 69,632 possible Final Fours - after yesterday that number became 20,736 - with various probabilities of occurring. 3.5% in that context? It's a lot. The next most likely outcome - three top seeds and Louisville - has only a 2.8% chance of happening. Any specific pick of four Final Four teams is unlikely. That's how companies can offer large sums of money for perfectly predicted brackets, without too much concern in having to pay out.
As for the fact that never has all four top seeds all advanced, well, that's true. But three of four had never happened either, until 1993. Now, it's not looked on as that rare. Folks are getting better at seeding these things, so it's bound to happen sooner than later.