Vehicles are piped through PhysX, it’s not a native implementation and barely anything to do with UE4 to be honest. PhysX is pretty much the staple physics engine for a huge quantity of games. All game physics engines are approximations and inaccurate.
I’d also strongly argue that games like Star Citizen are in no way mainstream or even common, and a multi-planetary gravity feature - or even changing the gravity direction at all, is extremely game specific. Non-fixed gravity directions in character movement means more netcode, at the very least more complexity. Most games have fixed gravity, and the last thing CMC needs is more complexity to satisfy a handful of games that would use this feature at the expense of all it’s users.
BTW: You can do a custom gravity implementation in PhysX if you like, just need to pass a flag to the PxScene:
https://docs.nvidia.com/gameworks/content/gameworkslibrary/physx/guide/Manual/RigidBodyDynamics.html