Riding a horse is the simplest thing in the world. Just point ’em and kick. What’s so hard about that? And as long as you don’t care where you go or how you get there, no worries.
But we’re humans prone to having expectations and goals. And horses are sentient with thoughts and emotions of their own. Perhaps the first thing that horses and humans have in common is a dislike of random chaos.
So then the horse or rider might decide some sort of leadership is needed and that’s when training starts. You ask for something simple: Go away from the barn. Walk on the rail. Canter. But they don’t.