Why did native Americans frequently attack cattle drives?

Cattle can be slaughtered for meat. Since the bison where being hunted to the point the Native Americans couldn't hunt any for their own food, they found they could raid the cattle drives and steal some cattle for their own use.

It was also a way for young braves to prove their courage and bravery by attacking and fighting--ultimately with the hope of killing and collecting some scalps--with the cowboys driving the cattle herd.