While I understand what you're saying to some extent, the real multicrew fun should come from doing stuff together, not standing around on a ship saluting each other in your underpants.
While the ship is in flight, its not like there is anything to do with ship interiors or even when landed. Its mainly just an immersion thing (although nothing wrong with that per se).
There could be more mechanics added to make multicrewing a ship more interesting, whether it be for mining or combat or whatever, but that doesn't require interiors either, just enhancement of existing multicrew.
You bring up roleplay, and again, nothing wrong with it per se, and doing in within ships, fair enough, but how many players are into that? How many want to spend their time standing around saluting each other in their underpants? (or whatever their roleplay consists of).
The fun for most i would say comes from actually working together to achieve some sort of goal. There are no goals provided by interiors.... unless FD add something like ship wreckage that can be boarded to obtain something. But own ship interiors.... not so interesting.
So, immersion, ok, roleplay, for those who are into it, ok, but the value for actual gameplay is limited.
Not saying they shouldn't do ship interiors, but they do have to balance the cost of implementation vs the added value.