You must not watch behind the scenes very often.....most films, especially scifi films have a magnitude of Previz and conceptualization behind them. If they were to have used every idea the film would have been 5 times the scale and10 times the length.
Yes, most films do have a magnitude of concept art and obviously you can only settle on one version for a specific set/idea at the end of the day.
However, I'm going to have to disagree with you with regards to the scale and length. They definitely had the space for sets of those scales. And a resourceful team can easily put together something that closely resembles concept artwork without it affecting the budget in a big way.
How would it affect the length, out of curiousity? Remember, a lot was cut from the film. Not only that, many of the sets and ideas had multiple versions. Everything I've seen so far, behind the scenes, has related to the film in some way.
I just think they made some poor choices on which concept art to adapt a lot of the time. Sometimes they did settle for the best version. But most of the time, I prefer the concept art.
As for other films - it's not always a case of the concept art being better.