The Atrophy Theory suggests that females are polygamous in nature because their interest in the relationship decline the longer they are with 1 man. This happens across both genders but in females this effect is highly accelerated when compared to males.
In physiology, atrophy refers to degeneration of cells and muscle tissue over time.
Divorce & Flaky behavior
The Atrophy theory can also explain why most divorces are initiated by woman. Even when they are married to top celebrities like Brad Pitt or Johnny Depp, there is also the issue of flakiness, which is very typical when it comes to female/male communication. Females are prone to momentarily 'disappear' when their interest gradually fades over time.
Evolutionary Biology & Possible Explanation
It is not known why females are so elusive. But according to evolutionary biology this might be a defense-related mechanism.