How valuable in the long term do you think a classified mailing list for San Francisco is to the world? Or an online bookstore? Or a social network for a school? Yet Craigslist, Amazon, and Facebook morphed into amazing things. YC companies do things that are tractable and fundable within 3 months, which is the first test. Then, with some focused success, they grow/expand. At this point, it takes some vision to imagine what these companies COULD be in 7-8 years (which is the average time to liquidity for venture investments).
Three outliers does not a trend make. I'm surprised you didn't throw google in there as well, since that one instance is all everyone ever talks about.
In 2000, everyone thought they were an Amazon. A boatload of people were wrong about that.
