Dirt tracks that we have today are nowhere near \"natural\". Horses run on turf. That is their natural surface. That is what their hooves and legs were designed for.
Horse racing started in America by racing \"from here to there\" (across country, across fields, miles), then developed into short bursts down city streets in town. Then regular racing fields (locations) where horse owners could gather to compare their stock and gamble.
Dirt was created because you run horses over turf a few times, and it obviously gets completely torn up.
Aside: England has figured out how to keep turf courses intact for a couple centuries, without going to dirt courses, but Americans couldn\'t - I don\'t know why. Something to research. Might have to do with the climate differences and farming/field/forest differences between the countries a couple centuries ago, but I digress ...
But torn-up turf with holes and divots breaks ankles and tears tendons. So somebody took a disc to the turf where the horses ran, like a farm field, to break up the clods and dangerous holes. Then harrowed it smooth.
Then the clubs in charge of keeping the course added in sand, for drainage, loam, for softness, clay, for firmness; all dependent upon the soil in the area of the country. Then the dirt was gradually deepened to the surprising depth it is now.
Dirt was the first \"artificial, created\" surface!