Many men are too unfuckable to meet women but too cowardly to buy a human-like doll. This creates a volatile storm of unsatisfied urges capable of overpowering a mind already fragile from loneliness. They start to see vaginas as the prizes at the center of mazes guarded by evil lady minotaurs. After a man spends too much time in imaginary vagina mazes, they decide they must be experts, so they write advice books for other aspiring minotaur hunters.
Some of the shyest, most unused penises are attached to men of sanity, but with enough self-importance to share their theoretical "dating methods." These authors are experts in sex in the same way a yellow belt is an expert in barehanded murder. They write basic, obvious advice for an audience that knows even less about romance, which is a demographic any census taker would describe as "N-no! This number is forbidden!!!" before vanishing in a puff of impossibility.
Some pickup gurus are certain that seduction is a metaphysical sport played with body language, mind powers, and subliminal messaging. These authors have absolutely tried hypnotizing someone into sex slavery, watched it fail, and then wrote a book on how to do it anyway.