Why were cars seen as 'green alternative' in their early days?

Horses would relieve themselves, on average, twenty-two pounds of manure a day, and the city’s production of horse droppings was estimated to rise to the level of the third story Manhattan windows by 1930. Stable owners would have to pay to have the stuff removed, and the result was that it often accumulated in vacant lots, which provided the breeding ground for flies.

The typical horse would consume about five acres of hay and grain, which is good for the rural economy, but bad for urban areas. Cars gave us sanitation, and they cleaned up the streets.