Let’s begin with a little History. The horse evolved in the Americas, but became extinct between 8,000 and 12,000 years ago. When the Spanish arrived on the American mainland in the 16th century, they brought horses with them and re-established the animals on the continent. There are about 9.2 million horses in the country.
I think Horses in the United States have significant popularity and status that is acknowledged by a number of observers and researchers. So the researchers was conclusive and it allowed to the United States to become the world's leading country in terms of horse births. In addition, the number of ranch and stable are very numerous.