I have to agree it's who you know. I've always thought my dogs could be perfect for movies or commercials. It's not a lack of training, they could knock it out of the park fairly easily.
But I don't live in LA or NY and I don't know the agents who book those things. By contrast, another dog trainer I used to know trained the personal dog of a TV producer and because of that her own dog was in several sort TV spots.
You need connections, unfortunately. I imagine the same would be true when it comes to horses.