I would imagine its because in the wild they walk over terrain where their feet are worn down such as rocks and other rough terrain.
For domesticated horses they stay in nice grassy paddocks or in stalls where there are fewer chances of their feet being worn down.
Also, domesticated horses are actually used for riding, etc. where wild horses don't have anyone riding them.
Horses feet actually grow like fingernails and if they aren't trimmed they will actually curl up and permanently damage a horses feet and legs.