People also used oxen and mules for personal transport (we don't even have numbers for these and the equine count is probably less than accurate as well), back when the population was less than 70 million (and couples had upwards of 8 or 9 children frequently, so much of the population was younger) before mass migration at the turn of the century (itself made possible by advances in transportation with steam engines in use at sea and by rail). By 1920 cars were already commonplace, but either way, most of the population was doing subsistence farming in the late 1800s and went to town maybe once or twice a month for supplies. We were much poorer before cars, and raising and maintaining a horse requires land and resources.

You can see SF in 1906 as cars started to take the place of horse and buggy: https://www.youtube.com/watch?v=sHkc83XA2dY. The early automobiles even fit the same profile as a buggy.