Because humans are genetically predisposed to it. We are socially monogamous, but not sexually monogamous. We will pair bond to raise our children which is being socially monogamous, but that doesn't mean that we necessarily pair bond with that person sexually. Which means that we will still seek out multiple partners for sex. This may be why a lot of people's relationships have "evolved" into wife sharing and open marriages.
Here's a pretty simple explanation...
Humans are socially monogamous rather than strictly or sexually monogamous. This means we tend to pair-bond and raise children as a team, but we also retain biological and behavioral tendencies toward extra-pair desires and serial relationships.
Experts in anthropology and evolutionary biology view human mating patterns as a flexible strategy shaped by both biology and culture.
In other words, sexual monogamy has been "forced" upon us by society, morals, ethics and ideology!!