To my mind there is quite a straightforward explanation. In order to profit from the initiative granted by the first move, White has to make use of his opportunity to do something before Black has an equal number of opportunities of his own. However, to do this, he has to make 'contact' with the black position. The first point of contact usually comes in the form of a pawn exchange, which leads to the opening of the position. … So the thought behind 1...c5 is this: "OK, I'll let you open the position, and develop your pieces aggressively, but at a price – you have to give me one of your center pawns."
— Jonathan Rowson, Chess for Zebras: Thinking Differently About Black and White
Between "not studying the opening" and "you should already know this" there are a lot of steps that you miss.
I didn't ask the question to learn the opening myself. I was wondering if the book that I have in mind already existed.