Is this a bit Silicon Valley Culture? Because those guys do the same - they have a software idea and work on it individually or with 1-2 co-founders. Why? Why not start an open source project and invite contributors from Step 1? Why not throw half-made ideas out in the wild and encourage others to work on them to finish them?

For one thing, because open source community isn't terribly likely to embark on a random poster's new project, and you'll end up developing it mostly by yourself anyway. Furthermore, there's this aspect of hacker culture, and especially open source culture, where it's actively anti-evangelistic, and dislikes developing user-friendly things like Ubuntu, preferring Slackware or Gentoo.