I'll tell you why no one has done an objective study
Because studies cost money, and people simply do not like to spend money on something that may not support their agenda. How would the greenies feel about spending money on a study showed that a steel-shoed horse that weighs as much as an Electraglide causes more trail erosion than a dirt bike? How would an ATV rider feel about spending money on research that shows his donut-tired fun machine destroys ground cover quicker than a herd of sheep? Both would feel pretty stupid. Much safer to spend it on lawyers to throw the wool over lawmakers' eyes to get their way.