Because in theory, they are in competition.
According to a post i read yesterday, 99.9% of dudes ain't about shyt....now that leaves .1%
Now .1% of the male population is roughly 153,000, so 153,000 men are considered good men.
Ok, so with that number, you have to consider that at least a quarter of those men are fat, or good guys, or some other type of man that it's been found that women don't particularly care for. Which leaves a rounded up 115,000.
That leaves you with 157,244,385 women vying for the love and affection of 115,000 men that are considered "desirable". That would create a cutthroat, hazardous environment.