Because it's never been done before with ED's unprecedented scope, literally billions of possible locations which already works for spaceship flying. i.e. the holy grail, massive openworld plus detailed freedom. StarCitizen tries to pretend it's there, but it's still stuck in one solar system of a handful of moons, planets, and still composed of illusory cryengine levels.
All other games with fp view, have very limited locations for the walking. There's always a tradeoff for massive scope (and framerate/playability) vs. fidelity & detail. Even if there was walking in ED's stations, there's heck of a lot of work in how to limit the walkable locations, and all the goof-ups, and possible grame crashes, and framerate drops that could occur with trolls pressing the boundaries, persistence not just with other player cmdr's assets, but your own ship and npc assets when in walking mode all within the current framework, and probably a myriad other technical challenges us non-EDdevs are unaware of, etc.
Of course I still have faith FDev are trying to work on it and it will take time and arrive eventually. I'd bet they've been working on it to some degree or another at least since initial multicrew & holo-me.