Is it because it is more rigorous than books like Halliday and Resnick? Or maybe you need to digest those books as a prerequisite for the feynman lectures? Or maybe people don't believe a regular person could digest the feynman lectures as a introductory level physics book? If someone is a beginner looking for a challenge on physics, would it be best just to read feynman, and to also do the exercises for the FLP? Thanks, your help is appreciated.