Because women in antiquity and the girl next door KNOW to go for the balls.
According to a recent article online, this may be so.
"Solomon Joseph Solomon’s ‘Samson and Delilah’ (1887) furthers this anxiety by displaying the scene almost as if it were a castration; a crazed looking Delilah gleefully dangling Samson’s severed manhood in front of him. Indeed, the entire story of Samson and Delilah reeks of castration anxiety and the idea that in a moment of male vulnerability, a vengeful woman with a razor can render a man permanently impotent. Regardless of how much power men wield over women physically and socially, a man who loves and lusts after women will always be in some sense emotionally and sexually at their mercy.
Carol Smith (1997) puts forth the idea that power is itself the theme of Judges 13-16 and that Delilah’s story should be viewed through this lens. The central idea of the story, that an otherwise invulnerable man can be brought low by a woman who is his lesser in the gender hierarchy, is just one example of power dynamics at play; the Philistines hold power over the Israelites, and their gods Dagon and Yahweh struggle for dominance. Delilah’s story is remarkable precisely because it shows that she accomplishes what the Philistine men were unable to in subduing Samson. The later “filling in” of details in the story actually enhances this narrative; by making Delilah a sexually seductive prostitute, artists and storytellers have played on male fears of emasculation and humiliation by a socially inferior yet irresistible woman."