It's important to remember that the economic forces that have made car ownership so widespread in the present did not exist in most of history, and an entirely different set of forces governed the likelihood of horse ownership or access. Be careful about generalizing too much from the present.

Edit: not that you were explicitly doing this in your post, of course. But I've found that some people tend to assume that, since cars "replaced" horses, then there were as many horses roaming the streets back then as there are cars today.