If you don't "get" it, then clearly you don't understand the question you are asking.
Why can't they "just" insert a generic FPS game into another totally different space ship simulator, with highly optimised peer 2 peer network code and no existing human-scale modelling?
Different networking requirements.
Managing human players "loose" in stations (can't have FPS CMDR walking under landing ship).
Human scale modelling & texturing of all ship and station interiors (some ships are done, but that's still monumental amounts of work).
Something to do on foot ("just" a whole new game) - major issues there. Just chat? Trading? Combat? Full RPG? See you in another 10 years...
Make sure VR players can use it (locomotion and motion sickness considerations).
Many players DON'T WANT or care about walking around. They'd find is pointless and boring.
Many players would prefer instead to see atmospheric worlds, water worlds, worlds with living cities, worlds with alien flora and fauna. All of which is more easily achievable with the current player model and game engine (Ship/SRV). And could be rolled out procedurally across 30% of all planets and moons in the galaxy (millions upon millions of undiscovered places to visit). The alternative being a handful of manually created environment "maps" outside of the generic station interiors.
For myself, I would prefer to see a phased introduction of atmospheric worlds and gas giants. That would give us plenty to do, while Frontier takes the time to write, or even re-write the game to allow a meaningful Extra Vehicular Activity (EVA) function that players can engage with.