The entire frame of a horse trailer is designed to a different standard. The load from a horse is carried to the trailer frame as 4 point loads; the 4 hoofs. Each in a small area.
This heavier frame is carried to the goose neck and resist the transmission of twisting forces to the rest of the trailer.
Add to that the benefits others have pointed out (empty bed, articulation, etc) and the popularity with ag types is easy to see,