Game engine implementations have different tradeoffs, and generalized game engines are optimizing for the lowest common denominator. Niche use cases often require custom engines. For example flight simulators need good terrain level of detail streaming, and may need to use special techniques to work around loss of floating point detail when rendering at a global scale. I don't know of any game engine that supports spherical height maps.

On the other hand Kerbal Space program is implemented in Unity, so maybe even off the shelf engines can handle that.