I am still wondering why there is no widely used open source roleplaying system. Most of the time the community is better than the developers in creating a rule system.
There is. Its called the D20 system, and there is a lovely SRD for it and everything. Problem is, it is based primarily on fantasy games.
As for other genres, you start going into niches and areas where mechanics start supporting specific settings and world views, such as the way Shadowrun interplays man, machine, and magic, or the way Vampire: The Masquerade promotes a certain style game. The most open games, other than 'quasi-freeform' (pure handwavium) systems like the Window, would be things like GURPS, Hero System, or Mutants & Masterminds, which have basic rules that can be adapted to many different settings, but trying to hammer out Shadowrun's Matrix in Champions would be a major headache.
Of course, you're more than welcome to do the work yourself. But most likely you, like me and many others, don't have that kind of free time and energy to devote to writing up a game system from scratch.