Why are we all arguing about this if there is already a mod that implements N-body for KSP? It even positions celestial objects according to the calculations, with some changes because the stock system is unstable.
Principia is great, but it also shows exactly why this wasn't done in the base game. It has a significant performance impact, especially at high time-warp and thus doesn't scale that well with many crafts. They're able to push those calculations across all cores, but that only helps so much.
https://github.com/mockingbirdnest/Principia