Part of the problem is that writing a script is a logistical problem as well as a writing problem. You have to account for actors maybe not being available, sc

Part of the problem is that writing a script is a logistical problem as well as a writing problem. You have to account for actors maybe not being available, scenes getting cut, action scenes being shot early for various reasons which means you have to retrofit them into the story you're writing, some actors being so expensive that any retake you shoot with them has to be justified, etc. In a book you can afford to write a plot where every little detail matters. In a big budget movie, you need to keep to the broad strokes, because course-correction is expensive.