I'm struggling to get through TFaS, really don't understand how it became such a 'PopPsych' (if that's a thing) classic.

It's like a collection of the obvious, the wrong, and autobiography of someone you haven't otherwise heard of.

It'a probably obvious to you because of cultural osmosis.

Of that class of book, the game is to pretend to have read it. A little like quoting a play having read a book of quotes.