historically humans were not monogamous like we are today. So why is lifetime monogamy now seen as the default?

The same with polyandry.
I think it's just about selfishness.
We're not meant to have one partner. Look how quickly people get bored in relationships. No matter how loveable you are, your partner will need a break from you at some point. If only humans could understand that, they wouldn't be so harsh on infidelity and we would have a lower divorce rate.