But it's not hard. Especially since KSP would be a restricted n-body problem (only the spacecraft are affected by gravity from multiple bodies) and the "n" in n

But it's not hard. Especially since KSP would be a restricted n-body problem (only the spacecraft are affected by gravity from multiple bodies) and the "n" in n-body is about 20 (number of massive bodies in solar system). It's a game design decision not to include gravitation from multiple sources.