Why are they called cowboys ???

Indeed to everybody who said that there's a distinction amongst horsemen and cowboys. Cowboys are a subset of horse riders and a considerable lot of them are bad horsemen. There are bunches of horsemen and equestrians who have never worked a cow or even ridden a western saddle.
I've read that the term "Cowboy" started as a kind of sideways racial slur, on the grounds that a considerable lot of the ranch workers in the old days were freed or escaped slaves, seeking a life in the frontier where a man could be treated (more) like a man, regardless of his skin. The name "boy" was still used on them, however. Like such a large number of parts of history, as the activity turned out to be more romanticized, the origins in racism are obscured by more racism, and every TV and movie cowboy miraculously turned white.

People who ride horses are not called cowboys; they're called equestrians.

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