It helps if you realize that most of the trails we have in the Western USA (not sure about other places) were built to facilitate horse or mule packing. Trails to fire lookouts were intended to allow mule trains to supply those lookouts.

In the 20s and 30s recreational trails were built so hunters and fishermen could access the forests, lakes, mountains and rivers of the backcountry and almost all of those early hunters and fishermen rode horses.

The PCT was conceived at first purely as an equestrian trail and its specifications were designed to make horse packing easy. Who else, they reasoned, would ever attempt a trail that long?

FWIW, we recreational hikers came afterwards and were not considered significant users of trails until after WWII. I have a Sierra Club book from the mid-1950s, Packing Light with Burro or Backpack that reflects the shift as it was taking place.

Which is not to say that your observations about the relative damage done by horses are inaccurate. It's just that, in a real sense, they were here before us and helped make the trail system possible.