I think it's because we, as humans, decided to make the concept of "marriage" up. Not only for our own selfish desires of "claiming" someone, but it is obviously turned into a huge industry, with transactions happening even in divorce.
Our natural biology fights us over having one lifetime partner. We have the desires of a Male lion with a pride of females.
The old kings weren't wrong with having harems. They were just going along with their biology. Marriage is a fantasy from a species who thinks too highly of themselves, and they wonder why it fails so much, and why people cheat.
Marriage is a contract and was created to provide support to the children, both in family bounds and societal order. Later in life, those children would provide to the elder. Was about survival.
Later, with abundance, daycare, wellfare, "equality" of sexes and the such, marriage became more of a egotistical romantic fairy tale, where children are a expensive commodity.
Nowadays, with skyrocketing high cost of living, children are used (with the help of the law) by women to provide for themselves, at the expense of the children and the father.
