Expensive horses take expensive breeding, training, and maintenance. You can get a good draft horse (one bred for real work) for not much. And "hobby horse" isn't just a fun phrase.

Horses used to be plenty, then automobiles pushed them out of their role in the order of things. They're the OG victim of automation at a species level. I imagine at the time they were making those movies, there was still a huge overpopulation of horses relative to the use humans had for them, so you know... supply and demand.

An average used car costs a couple thousand dollars too.