Also, not everyone understands what good content is, or how to properly markup their websites.
The field of SEO is quickly (or slowly depending on who you ask) being transformed into a much more holistic discipline where content creation, usability, conversion optimization, pr and marketing are falling into SEO's job duties. It's much more than page markup and links.
Mind telling me how a consumer web startup is supposed to get others who are interested in X to visit his site about X? This is the basis of SEO: matching searchers to pages. It doesn't happen magically just by wishful thinking, and writing good content unfortunately.
But because that's the case, and you got a quality product, you're saying to sit tight, don't get out of the building, don't try to sell... and hope people will eventually choose you?
So how exactly is a consumer startup going to get people who want to know X to visit your site about X?