Because that is the way that we evolved. Consider that are closest genetic relative is the chimpanzee which we share a common ancestor with and they are one of the most violent and tyrannical animals on the planet, and great apes in general are characterized by constant infighting and power games within their groups.
Since women produce a limited number of eggs throughout their lifetime and gestation leaves females vulnerable in addition to the high investment of time and energy raising young to maturity, women evolved to be very discriminatory and to be constantly evaluating the viability of a chosen mate at any given time rather than run the risk of being stuck with having to carry and raise the offspring of a genetically inferior mate. Also, because of the high investment that offspring require, women also want to be in a position where they can have access to all of the resources they need for both themselves and their current or future offspring and if they cannot get them from their current partner they will find another one that will. Plus, even if they find a mate in a powerful position or who provides optimal conditions or resources, there is always the chance that he might not necessarily be the best from a genetic standpoint. So, this is also why females of many great apes including humans are constantly looking for better mating opportunities behind their current mate's backs even if they run the risk of getting caught because there is a chance that they can be impregnated with the offspring of a superior mate while taking advantage of the conditions that their current mate provides.
This entire phenomenon was summed up by the famous social anthropologist, Robert Briffault when he came up with Briffault's Law:
"The female, not the male, determines all the conditions of the animal family. Where the female can derive no benefit from association with the male, no such association takes place."
This is also why men are constantly fighting with each other or backstabbing each other over women, since the mating strategy of males in most great ape species including our own is to try and build up a harem and impregnate as many females as possible as sperm is constantly produced in huge quantities by the male reproductive system so the behavioral drive for men is to mate as often as they can in order to increase the chances that their genes are spread.
I want to emphasize that almost all of this behavior is entirely instinctive as most people lack the self-awareness or wherewithal to examine why they act the way that they do, especially when it comes to partner selection, mating, infidelity, and children. I do not think that either men or even women consciously set out to be "evil" per se, just that we are captive to our instinctive drives which evolved in a world that no longer exists as neither genes or evolution care about the happiness of creatures as individuals as they are driven to reproduce successfully under optimal conditions even if they damn themselves in the process.