That'd be totally unrelated to the engine. Regular physics engines don't handle that kind of stuff. They only handle collision. It's up to the studio to implement gravity, and it's they're decision on how that's done. There's no reason n-body physics couldn't be supported. It can be in KSP even. It's just significantly more calculations to perform, hurting performance, for a very insignificant (usually) effect. It's also much harder to play, teach, and learn. I would say a smart studio doesn't do n-body by default, but maybe create an option for it, though this requires a totally separate gravity implementation.