There is "game lore" detailing why CMDRs are not allowed to exit their ships on stations and atmospheric planets with life, supposedly because of the chances for "contamination" and I think that is an acceptable excuse but, it does not explain why CMDRs cannot leave their seats and move about within their own ships. As I understand it, the Anaconda is the only ship that has anything approaching a fully modeled interior. I think the reason there are no "space-legs" even within our own ships is that FDev does not want spend time and effort to model the interiors of all the ships.
To be fair, properly mapping the ship interiors would be a gigantic undertaking.
I think most of us are aware that the estimated sizes of the core modules and optional modules would
easily fit inside the ship models we currently have.
Physically fitting, say, 3x C4 slots (probably around 16m³ each), 3x C2 slots (~4m³ each) and a couple of C1 slots (~2m³ each) into the hull of a Cobra, along with the core internals (which wouldn't actually
need to adhere to the volumes used for internal slots) would be a piece of cake.
The hard part (the really,
really, hard part) would be making it happen in a way that always produced a plausible ship layout, regardless of what configuration of modules a player chose.
For example, if the interiors were just arbitrarily designed with physical spaces of suitable sizes in preset locations, a player could choose to fit a C4 SRV hangar and it'd end up stuck somewhere which meant there's no apparent way for the SRV to be delivered to the cargo hatch for deployment.
Or, perhaps the interior of a Beluga is designed with a luxurious interior that includes corridors with red carpets and gold fixtures... and then a player fills all the interior slots with cargo racks or mining equipment which, again, would have no apparent connection to the cargo bay or cargo scoop.
Ideally, the whole thing would need to be completely "fluid" so you could bung in whatever modules you wanted and the game would have to figure out a plausible internal design that catered for all your chosen modules.
More simply, perhaps it might be best to just allow players to design their ships manually, so the game shows you a plan of the ship and it'd be up to you to drag & drop modules to place them wherever you wanted.
That way, at least, if you ended up with a 1st class passenger cabin stuck between a refinery and a limpet controller, it'd be your own fault.
I guess the outright easiest way to model ship interiors would just be to ignore internal slots completely and only ever grant players access to a small area consisting of, say, crew quarters, a mess and, perhaps, some kind of systems control room.
That'd be a bit of a cop-out and a lot of people would probably be salty about it but it
would be the easiest solution.