That's not necessarily the case. There are many niches that aren't obviously profitable. From the outside, it seems like they'd be a waste of time until someone discovers a way to make money with them.
also, certain niches may have limited time windows where it's relatively easier to make profits and/or there are no competitors active in it or only a few. after this window, profits are driven down to commodity levels and/or a large number of strong competitors enter the same space and bite off some percentage or all of the market that was previously yours
It'd be fun if, every once in a while, a post was just dropped on HN pointing out a time-limited niche as it begins. The result would be somewhere between a flash-mob and a gold-rush: hundreds of competitors popping into existence in the same space over the course of a weekend.
