I think there is so much in gameplay and assets to work on that is easier, it makes sense ship interiors are lower priority. Interiors potentially also have a high annoyance factor and lower utilization than planet and outpost/ station interiors. Repetitive long walks down the same corridor in an Anaconda would get pretty boring after a handful of trips. The game cost and complexity of issues dealing with physics and object counts when walking around in space also probably taking more R$D.No offence, but why do so many people feel the need to point this out?
"It's a space game, you can't do anything or go anywhere, there is no meaningful gameplay, but you're in space and the devs thought that would be sufficient." :-/
Simply walking about or even shooting stuff in a building, on the ground, or in a space station that isn't moving like spaceships do = easier; even with some +/- modifiers for different gravity. I don't say easy because they still have to work all the new assets into the PG system and develop the gameplay to work in the game they have already built.
Either characters would be required to be strapped in during flight or a total handwavium pass would be required. I wrote about this issue before, saying for passenger ship gameplay that ED should include "passenger comfort" instrumentation and ratings. Commercial airliners for example can make stronger maneuvers than they do, but would severely freak out the passengers. Also, spaceflight adds even more complication. With no artificial gravity, ships are zero-g in space. But even with artificial gravity it would be hard to correctly model characters moving inside flying ships unless total handwavium is sprinkled everywhere and humans are considered indestructible an amazingly strong. Why? The models are going to be subject to many strong changes in thrust and motion. During flight, particularly any with rather "violent" maneuvers, explosions and projectile hits. Do we and FD handwave and ignore all the thrashing about the inside and say magnetic boots are sufficient to stabilize a human body and prevent being banged against all the walls? Because the boots only hold feet to the floor, and don't stabilize the legs, torso, arms and head. Once FD opens up walking in ships then that issue, amongst others like handling ship hijack/theft, has to be addressed.
Having said all that, I do want to see ship interiors as well, and no doubt they can be modelled. So hopes for the future.