The distinction you make between "professors" and "textbook creators" seems strange to me here. At least two of the best textbooks I've seen were written by a professor who was teaching the class and wanted something different than the other books.
I think this is the ideal way for them to be written. The idea of having separate "textbook companies" seems more like what happens in high school, and it produces pretty bad textbooks.
I agree. I was actually imagining that, say, the history department at some school would team up to write a textbook and publish it. What better way to build your department's reputation?
You're right; departments might do it for the prestige. And of course it might make their teaching easier to have a textbook customized to exactly how they teach the course (after the initial work of writing it).
I think this is the ideal way for them to be written. The idea of having separate "textbook companies" seems more like what happens in high school, and it produces pretty bad textbooks.