2) If so, what in your opinion accounts for this?
You won't hear too many themes in Bernard Herrmann's scores. Or Jerry Goldsmith's. You will hear minimalism and serialism and huge amounts of different time signatures and rhythms. . It's a question of what a writer likes and is comfortable with. A lot of writers aren't comfortable writing themes and otoh a lot have to have a theme just to write around it and get kick started. There is no one account for anything.
Where a lot of film music may sound rather ordinary, is not so much because of theme(s) or the lacking of themes, but because the lacking of rhythm or coherent orchestration or bad intervals or minding numbing repetition of ostinato strings. Or time constraints (as in the amount of time they have to write). The music of HZ is like conveyer belt music. IT'S NO GOOD a teenager telling you their dick grew 5 inches on youtube JUST BECAUSE he likes it and its easy for him to understand. Just because someone likes a thing stultifies any objective conversation more or less about anything. Music, books, art, films, photography, surgical procedures....anything. I might as well argue why I like blue better than red. In the end, you either try and understand what works and why something else becomes irritating on some kind of intellectual level or you're dead from the neck up. Or like Brad, you become a Rock Star.
I believe film music is a done and dead genre and has now more or less been covered over the last 80 years. What else can be written that can be construed as original? Why do you think chugging strings are the order of the day now? Because films are made from the bottom up - not from the top down. Finance. And financial considerations when making films are not aimed at people that could be construed as intelligent. Intelligent people don't sit in cinemas nowadays - eating anything - wasting their time and money watching drivel with a constant low pounding string noise. Unintelligent people do that and they will always be the target market for just about anything because historically they've always been an easy sell- for anything. It's got nothing really to do with the music.