In the book, why do you think that the author explains all these tests and procedures they have to do at the fertility clinic, but he never says what is wrong with them?

This should explain it for you...... there was no detectable reason for their inability to conceive a child.

"After months of trying to conceive, Amir and Soraya consulted fertility doctors. Neither of them had any detectable fertility problems, but they were still unable to have a child. When they told Soraya's parents, General Taheri and Khanum Taheri were disappointed. The general urged them not to adopt, most of all because Afghan society depends on the line of succession, which the act of adoption obliterates. Amir thought privately that his and Soraya's infertility was punishment for his betraying Hassan so many years before. Soon after they discovered they could not have a family, Amir and Soraya bought a house. Despite their newfound material comforts, the absence of a child tormented them both."