YC is meant for very early stages (with an idea and not much else). Stripe wasn't worth much when it was just an idea--a better payment gateway--but is now worth a lot because they executed. YC wants to help teams better execute and starts things off with enough money to let the team live in the Bay area for a short time.
Many (most?) of the companies that presented at the most recent YC demo day had substantial revenue, and many of those were already making revenue before they joined YC.
(That's a big difference from the YC batch I went through back in Winter 2011)
