When formatting a large document, those costs are amortized. In addition, many changes are set-it-and-forget-it. Once you set the right dimensions for an image, table, header text size, or margins, table of contents layout, bibliography style, etc., you wont have to mess with them again.

In a word processor, an image size can be changed by inadvertently clicking on something, the style of a text selection can be changed by moving the mouse scroll wheel on the font drop-down menu, page numbering may not refresh when working on a large document, inserting a figure may screw up table or image numbering, etc. You have to be constantly checking your multi page document to make sure everything is still ok.

As a teacher, my heuristic is simple: if I'm authoring a document that is less than 3 pages, I use Word. Otherwise, I use LaTeX.

For example, I write exams and syllabi in LaTeX. Quizzes, assignments and handouts in Word. It has worked well for me for a decade.