Why would anybody with a good idea for an app give it away on a forum?
Imagine Mark Zuckerberg had posted his idea "A site where you can like the girls on this university" (see the movie) on a forum in stead of making it himself ... The difference for him would be many billions of dollars.
Or the founders of Rovio would have answered "Something with angry birds maybe?" in stead of, well, you understand why they didn't.