It is actually a resource thing. It depends on how many people, how much time, how talented, money, etc.
As to your second point, I don't think it's true that other AAA games aren't doing this(don't know if that was what you meant).
I would argue that RDR2 and Naughty Dog games are there in terms of putting this amount of care and detail into their environments, but Naughty Dog does it very linearly and Rockstar has lower quality in terms of rendering of objects up-close.
Star Citizen is trying to do both, but at the same time not doing it better than those two as a consequence.
The big difference is, the other games actually release after 5 years. SC is trying to do all at once and as a consequence has many smaller working elements that don't come together yet as one big game. In SC's case, the whole is smaller than the sum of its parts.
Rockstar and Naughty dog already had fully functioning dev teams before starting those games. Not to mention that their creating their games on the back of previous games. Anywho what SC is trying to accomplish has never been done before so it stands to reason it would take longer than most AAA games.