Wouldn't you think a horse could go fast enough under its own steam?

In fact, the faster the horse, the greater the need to have it go at speeds that no horse could possibly attain under its own steam.

Many an average equine plodder will spend its entire life on its own four feet plodding its way happily from  A to B and back again. That's as long as both A and B have a goodly supply of grass or other good foodstuff. 

But if that equine has a swift turn of foot, if it can gallop for miles at high speed, then suddenly there is a need for it to travel much faster, say 60 mph for hundreds of miles as it is taken in the back of a lorry from racecourse to racecourse. Or if it's exceptional in the speed department then it gets to travel at many hundreds of miles per hour in an airborne stable.

And there is money to be made transporting horses, even when you need to employ six other horses to pull the trailer.