@lain being good at a topic ≠ being good at explaining it. especially in academia, where the professors with the highest qualifications are poached by universities for the research opportunities (and they're forced to teach to maximize the uni's investment). usually, those people aren't really given any pedagogic training, and are just dropped into a classroom. very rarely will they be a naturally born "good teacher", and this subsequently applies to textbook authors who are usually less qualified than the top academic brass (depending on the field ofc)