Humans can trample each other too. Horses don't naturally and normally trample people. They can and do occasionally, but it's not a natural inclination for them. Most of the time a horse running over a person will just step or hop over them. Horses are usually very careful about where they put their feet, since breaking a leg isn't a good thing for a horse (not that it's fun for anyone).
If a horse tramples someone, rather than kicking or biting them, it's either an accident, like because they're running in a herd and don't see them, or their rider makes them run over someone. Usually that requires the Trample feat.
