Why don't girls have balls?

They sort of do, in a biological sense. Testicles are where the male produces sperm. In the womb, during gestation, all humans begin as female. It's the combination of hormones that cause physical changes into the male anatomical configuration. The analogous structures to the testicles are the ovaries, where women produce eggs.

A woman's ovaries stay inside the body. But a man's testicles descend, usually not fully until puberty when they begin manufacturing sperm. Scientists believe that this is because sperm are temperature-sensitive, and need to be in a lower temperature environment to be healthy sperm. Since the scrotum hangs outside the body, away from the body core, the temperature there is lower and makes an ideal environment for the sperm cells.