Why in the world do men honk at women running?

I hate the gym and exercise videos. The one workout I truly enjoy is running outdoors. When I go for a run, I wear comfortable running shorts and T-shirts or sleeveless running shirts. I am a sweaty mess and mind my own business, yet somehow people driving down the street think it is appropriate to honk or yell out their window at me. What in the world makes someone think this is appropriate or welcomed? It's not only rude and annoying, but it's potentially dangerous. Horns are so loud when they're right next to you and if it's unexpected it can be very startling. I've tripped from this at least once. What if I fell into the street? What is wrong with people?