I think lots of times these professors aren’t the best experts in the field, and thus it doesn’t make sense to use their books over other, better written ones.

Honestly, then why are you taking classes from "professors [that] aren't the best experts in the field"? Lol. Profs that write text books, usually write them based on how they teach a course, which is pretty much the best kind of text book.

a close friend had to purchase a $200 textbook on Microsoft Office written by solely his professor. It seems highly unlikely that’s the best instruction available.

Source?

I would counter that it might be the best text book for that professor, teaching that course.

Or at least they shouldn’t be getting a windfall from it.

Are you kidding me? Windfall? Do you have any idea how long it takes a professor to write a text book? Years and years. Do you have any idea how much money they make on publishing a text book? A couple of thousand dollars, over the lifetime of the book... and that counts reworks for new editions.

Yes, textbooks are expensive. Every college kid goes through it. But, they aren't expensive because the evil college professor is scamming money from cash-strapped students. There are very valid economic reasons why textbooks are so expensive, and none of them include financially selfish reasons by the professor/author.