I think it's more that it's the people who are on the fence about doing a startup that are likely to be on a startup-focused forum talking about it. The ones who know it's a good idea are out there founding companies. And the ones who know it's a bad idea are at their 9-5 jobs and don't pay any attention to the startup scene.

Never attribute to malice what can be explained by math.

(Edit: BTW, Drew & Aston used to be regular posters on Hacker News, as did drusenko of Weebly, coffeemug of RethinkDB, lacker of Parse, danielha of Disqus, aaroniba of Etherpad, and a bunch of other YC founders I've forgotten. When they realized it was a good idea to start a company, they quit posting, got into YC, and started building their companies.)