There were a number of thoughtful sci-fi movies in the 70s that did pretty well (Close Encounters came out same year as Star Wars). It's possible they thought that kind of approach would still be commercial.
Close Encounters is not what I'd call a thoughtful film. I find it really rather brainless, more about emotion and sensation than anything rational or logically coherent. After all, UFO belief is a modern pseudo-religion, and CE3K is very much a story about pursuing faith and mystery in defiance of logic and common sense. So it's more "feelful" than thoughtful.
I agree it is essentially a mystical story, but allowing for that it's still a more meaningful story than any of the Star Wars movies, in emotional and social terms. Its approach is quite low key, not an action movie at all (the hero is a schlub whose greatest physical feat is to climb a steep hillside).