While this may be true, most (good) textbook writers are very experienced teachers and writers which is the reason they have a book published and you don't. In addition, teachers have enough on their plate with grading, rubics, creating exams, meetings, and planning, why should they have to take on more work?
Furthermore, when a teacher leaves, unless they've left a complete curriculum behind, the new teacher has no idea what the students have learned exactly. With a textbook, the teacher can see the ideas as well as the exercises they have completed.
I can see the pros and cons of this, more pros I believe, I think that there can be a happy medium: use a book as a guide and then supplement as necessary.
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