Because, after reading, you are overthinking the whole process.
You should wonder:
- before you read the PUA-books, you had no fear of approaching. Was your approach succesful? I mean, did you hook up with the girls, or did you have only nice conversations?
- If you were succesful already (of course depending on your goals), ignore the books and go on with your own style
- If you are not satisfied with your game, see how the books can improve your game. Since you had no fear of approaching, you have overcome the biggest step already
