I can think of a few reasons why it was a pain to add at any given point, (and then you just have to imagine some dev inertia/laziness/other priorities to ge tt

I can think of a few reasons why it was a pain to add at any given point, (and then you just have to imagine some dev inertia/laziness/other priorities to ge tthe rest of the way): The game started off with only on body, Kerbin (earth knockoff), with the sun going around it. They then added the Mun and put in spheres of influence. As they added more planets, if, at any point, they had changed from the simple model to the true one, it would have broken all tutorials, delta-v calculations, extant saves, etc. It would also have required more changes to make the game properly playable. In KSP, players will often leave vessels in orbits on timewarp while focusing on something else- they'll have 10 missions ongoing at once. This works well when orbits aren't being perturbed, but in real life/real physics, you can't just leave a vessel in orbit around the moon for a year without course correction- it'll crash. The problem is, the game has no way to automate course correction/stationholding, and that would be far less trivial to add than merely changing the gravity model from one system to another. I'm not apologising for the devs not having done it right, really. KSP didn't really live up to all its promises, though it is pretty good.