Interiors don't even need justification. It's a game about space ships.
They absolutely do. I don't know why the type of game makes a difference here; for example, World of Tanks doesn't have Tank interiors, despite it being a Tank game. Why should Elite have interiors just because it's a spaceship game?
Ideas for interiors don't need minute detail, they simply need something that is enjoyable, practical, and by nature adds content that couldn't be better achieved some other way.
The fundamental problem is that most of the stuff that happens involving ships involves the ships doing things, not stuff happening inside the ships.
Take some real-life examples. If you have a tractor, you don't
want to have to fiddle with the inside of the tractor; you just want the tractor to do its job without breaking.
Or a boat; you don't spend much time inside the boat, because if you're using the boat, you're usually using it to go from one place to another. For the most part, large ships are just mobile houses, and there's a good reason most house simulators are like The Sims.
Now, if people wanted a sims-like ship decoration experience, that would at least be content I couldn't criticize. It's just got nothing to do with Elite, in my opinion. But that is, at least, is just an opinion, and the relative value of such an experience is one we could debate.