There are two reasons for no ship interiors:
1) Laziness/lack of creativity and effort.
2) Lack of skills
The very fact we get these threads and massive discussion shows how in demand ship interiors are. EVen though there are several forum whiteknights that will praise anything that FDEV say, I would say that large portion of the playerbase wants ship interiors. And a large portion of the space sim community would want ship interiors, SC is almost primarily popular because of interiors and they much more successful than Elite despite being barely a game. And whenever Elite and SC are in the same conversation, 99% of the time the first thing said is "But SC has ship interiors". And ship interiors would certainly have been preferred over 30 static plants that do nothing but sit there and you point a doodad at them to increase some meaningless rank that doesn't affect anything.
Therefore there is no argument that FDEV aren't making ship interiors because "The players don't want it", that is just demonstrably false by threads, polls and the wider space sim industry.
This leaves the two options above and it could honestly be either one because the only creative aspects in Odyssey are the cosmetic skins, nothing else is original or interesting, the settlements are detailed but nothing amazing and the exploration update is laughable, out of an entire galaxy we get 30 static plants that you can't interact with in any way.
After 7 years since the release the game has given us a barely functional powerplay, barely functional multiplayer, almost pointless fleet carriers, horizons which did provide a decent amount - the only decent additions outside of Horizons were thargoids and the mining update... and then Odyssey.
So FDEV are either too lazy to bother thinking up of decent gameplay features for ship interiors (of which there are many, just look at Pulsar or Star Wars Galaxies) or they don't have the capability. I think it's a mix of both.