I'm guessing it would discourage potentially good candidates from trying if the odds look too long?
So if we quoted numbers, competitors would be tempted to do bad things in order to get higher numbers, and then we'd either have to the same bad things ourselves, or look as if founders liked us less.
If you are congratulations for sharing so much of what goes on at ycombinator, you have been a great inspiration.
If you are not why not help other "clones" become better by opening up a little bit more about how you achieve your magic, kind of open sourcing technology "incubation" processes, so that people from the rest of the world can enjoy such an opportunity.