CCD is expensive in terms of processing power, and not to mention that it doesn't always work (I use Godot as a game engine, and after years I think it's not yet fully implemented).
There's a game that actually allows you to build your own ships/rovers/bases, and have walkable interiors, you can repair, or even build your ship wile moving. That sounds awesome and all, but it has limitations, by default max speed is 100-120ms. There are mods allowing higher speeds, but there is when you start having the tunneling problem, again, there CCD seems not to be implemented.
BtW, the game is SE (being s for space and the e for engineers).
I think it's not easy to implement, but it's not impossible, when we are in supercruise from other instances we are just a sprite, a dot, whatever. When we are parked somewhere it's not a problem either. The problem is only when flying in the same instance with others.
Not to mention, FDEV does not want artificial gravity, and tries (although with mixed results) to be realistic, inertia is the problem, if you walk around when the helm is dogfighting, you'll be dead in no time (except when flying a T9 or T10, were you won't notice if you're moving or not XD). An "easy" fix would be not allowing to move from any seat till the ship stops (in space or landed it doesn't matter).
Anyhow, another reason for the difficulty might be the amount of textures used, on foot/srv you have more detailed ground textures, but probably most of the planet is not instanced, when you're flying the planet is rendered dynamically... so in the end, you don't have enough ram to store all textures so the game won't run smoothly with all ship interiors textures plus exteriors. BTW, have you noticed that coriolis starports interiors are loaded only when you're very close?