In the early years of yc, people said there weren't any good companies. It took years for Dropbox to become Dropbox. There are actually a lot more great founders and companies now, but it will be years before you realize it. That's the nature of this business.

Your point about ambition is a good one though. Filtering out people who are just there for the prestige generally isn't that hard though -- they have ways of self identifying :)