A professor would rather spend their time teaching students and finding the best existing book (less expensive if possible, but better quality is more important) so they can use it for teaching.

I'm sure there are professors who have been thinking of writing their own textbook and getting students to buy them ... the downside is apparently writing technical books and textbooks earns less than minimum wage, and that's if you are lucky because of the way revenue split works with textbooks. So they have to charge exorbitant rates on the textbooks to make even a little bit of money. So most professors don't bother and instead use an existing good quality textbook. Not to say there aren't others who would try to write and sell their own.