True but it is quite a load of work. People on average change jobs about that often as well nowadays. It is like a massive project or a fulltime job for x years. Even Guido the Python BDFL largely is more of a Board of Director for Life. He developed Python largely on his own for 5+ years. Only so many years in a project per programmer, not everyone is a Linus or Guido. Too many other fun things to get into after the initial purpose/goal is achieved.

Python's code_swarm: http://vimeo.com/1093745