Here's why:
* Size and Structure: Horses have a relatively small mouth opening and a throat that's designed for consuming grasses and other vegetation. Their teeth are adapted for grinding, not tearing. A human is simply too large to fit in a horse's mouth or be swallowed.
* Digestive System: A horse's digestive system is built for processing fibrous plants. Their stomach is small and their intestines are long, designed for digesting tough vegetation, not large, complex animal matter.
While horses can be large and powerful animals, their physical anatomy prevents them from swallowing a human.