Well, YC was bootstrapped (marketing-wise) by the goodwill of pg's personal website too. That had plenty of same-minded exposure in the beginning.

Ten years later, people now see YC as "we can give you a better future/make you rich/famous/etc if you work hard and receive our magic touch."

It's a very word-of-mouth spreading meme. "This company makes you rich, be one of us, shub shub."