[confidantlake]

Every time there was technological advancement the number of jobs for horses increased. For example the invention of the railroad meant they no longer carried the mail long distances, but still the greater economic activity lead to greater demand for horse labor. There may have been some shifts in what jobs they did, but the trend was clear. More technology lead to more horse labor.

And then the automobile was invented. And over the next few decades the demand for horse labor tanked. Now the demand for horse labor is a tiny fraction of what it was a century ago.