The only thinking that helps me even begin to understand it is to imagine what it would be like to bring girls into a subsistence society with zero welfare provision. To eat and survive, those girls must marry - the only means of support they have is essentially the sale of their sexual and childbearing potential. Therefore you do whatever it takes to make them marriageable - otherwise they will at worst starve, or at best be completely dependent on a brother with zero status. God knows our society was organised in much the same way not very long ago. It just happens that this bizarre outcrop of torture became associated with marriageability at some point and spread in certain societies.
I read some more articles yesterday. In Somali society, there was a good level of knowledge of the negatives of FGM but a very high proportion of people believed it is a religious obligation. This makes me so angry but does offer a route of hope if religious leaders will participate in dispelling that view.
Incidence IS reducing a great deal - Somali societies are outliers in a way they weren't a couple of decades ago. All power to the Somali women fighting it.