The French book says that "the word ending in eur becomes negative" should be changed to "euse", so why isn't professeur negative? Why not change it like this?

Most nouns in French are different in yin and yang, but with a few exceptions, some nouns are in yin and yang forms. Words like professeur are French idioms. Professeur used to be a male, and there were few female teachers, so now only the positive form of Professeur is used. Similar words include mé decin, sculpeteur, dompteur, chirugien and so on. . . If you want to express the meaning of female teachers, you can simply use articles to distinguish them, such as: la professeur or une femme professeur. Don't be so rigid!