It all comes down to biology.
Your testicles actually developed in your abdomen near your kidneys. From there, they descended toward the scrotum, pulling the sensory nerves with them.
That means you have sensitive nerves that stretch from your stomach to your scrotum.
“So when the nuts are hit or squeezed, the nerve sends a signal to the upper part of the belly, which is why it hurts in the stomach when you get kicked down there,” says Darius Paduch, M.D., Ph.D., the director of sexual health and medicine at Weill Cornell Medicine.