Why does people call CowBoy?

Hahaha, now thats a thought. If you were seriously asking that, I seriously dont know the answer. Sorry..^_^

Originally, the term may have been intended literally - "a boy who tends cows" - but had developed its modern sense as an adult cattle handler of the American west by 1849. Variations on the word "cowboy" appeared later. "Cowhand" appeared in 1852, and "cowpoke" in 1881, originally restricted to the individuals who prodded cattle onto railroad cars with long poles.Names for a cowboy in American English now include buckaroo, cowpoke, cowhand, and cowpuncher."Cowboy" is a term common throughout the west

No, look at the meaning cowboy. Cowboy means : A cowboy is an animal herder who tends cattle on ranches in North America, traditionally on horseback, and often performs a multitude of other ranch-related tasks.

They are called Cowboys because they handle cows. Horses are just part of their equipment. Horse handlers are call trainers, or breeders.