Why Are So Many Film Motifs Harmonized Diatonically?

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?

I want to say because film composers don't get a lot of time to write overly "complicated" music, so they are forced to resort to that. Another reason could be because most people nowadays don't seem to appreciate harmonically adventurous music (if you listen to more "popular" pop music nowadays, the same simple harmonic progression is present in all the songs).

There's pop film music, and there's film music. 

there is much room for variation when writing diatonic music. Here's one example from John Williams since you mention him.

https://www.youtube.com/watch?v=kODnDpYTrqE