Because ruts never widen and move in mud and Roman metalled roads would be somewhat more resistant... Maybe the Romans carved ruts into their pavements... Table VII of the Roman Twelve Tables (a sort of bill of rights and responsibilities) laid down a standard width of 8 feet (Roman size) for the straight and double that on bends. I recall something along the lines of carts not interfering with pedestrian traffic, so carts passing would have to have wider roads which would need more maintenance - the responsibility of those who lived nearby. https://historycooperative.org/the-twelve-tables/ and others.