The main reason is conditioned reflex. Because humans often use some spoken words to express their meaning when taming horses, after a long transition, these spoken words will form a certain conditioned reflex in the horse's brain thinking. In layman's terms, horses will remember what a slogan stands for and then respond accordingly.

Foreign proverbs:

Giddyap is often used in European and American film and television dramas. The slogan' giddyap' may be due to the age, but many Americans don't agree with it. But it is not uncommon in film and television works. For example, how did I meet your mother appeared once in the fourth episode of the second season. When Ted was riding a horse, the girl patted his ass and shouted' giddyap' twice.

Besides, "Yes!" This is also a widely used form of address, and' yeah!' Another variant is "HYA!" It is also common in major film and television works, such as the romance of mom and dad in the fourth episode of the sixth season, in the lyrics of Marshall's Marshall vs. Robots.