You can find this answer by reading in Chapter 14. In that chapter, Diamond tells us that these kinds of institutions develop because societies come to have larger populations when they start to have agriculture.
When societies get bigger, it is harder to control them through informal means. When a society is small, everyone knows everyone and there are kinship and other connections between all the people. In such a case, informal control can work. But when society gets bigger, these controls no longer exist. Now, there is a need for more formal ways to (for example) prevent people who aren't related or in any way connected from killing one another. This is where government and religion come in. These institutions develop as a way of keeping peace and order within society as society gets bigger.