My guess may be that too composers these days have little experience with symphonies and orchestras and listen too much to popular music…? Or rely upon what amounts to a mix-tape of past works and call it a “soundtrack?”
They compose by themselves or with a small group in a studio with electronic equipment first and maybe bring in musicians to play needed parts. IOW, it may be too difficult or expensive to translate themes and motifs into a larger score that needs the enlistment of an orchestra?
…Although Ennio Morricone and Danny Elfman did get their starts by composing popular music. So that probably wrecks my hypothesis. JM.02