From what I have seen, most of the themes that film score composers like John Williams, Michael Giacchino, etc. write are almost always harmonized by diatonic chords, or chords that fit the key. Sure, every once in a while you see a chromatic harmony like a secondary dominant or something, but it's almost solely diatonic (I play these themes by ear on the piano and I know that I am not making a mistake when deciphering them by ear). With the vast array of ways to harmonize, which I'm sure they have studied, why do they stick to such basic progressions?