Ah, yes. Let me just point out first that most of us do not buy our manure. It is a free resource but sometimes transport has to be paid for.
As to using the manure, I'm sure that there are many areas that have lovely soil that do not really need manure but anyone working on heavy clay will know how quickly the soil turns to concrete without vast amounts of added humus. Horse manure is ideal because it is both free and low in fertilizers and therefore can be applied in large quantities.
Compost will work but there is an inherent problem of space required to produce it, never mind sourcing suitable ingredients. My compost factory occupies about one pole of my precious space and that would need to be increased 5 fold to come close to competing with the manure I use.