We had a pram for our ~ 15 lb dog as she got older and wasn't able to walk longer distances without dragging a leg or just tired but enjoyed going outside. She had a spine injury while younger and generally had good quality of life at that point. I've seen a few other small dogs in prams and chest slings in my US neighborhood. Perhaps the owners come from further outside the area via foot and don't want to carry a tired dog home? Japanese also seem to like accessorizing for their hobbies (see hikers with full pack + bear bell on 2 hour busy trail)

I find some of the other things around Japanese pets puzzling like why are otters kept as pets https://www.nytimes.com/2019/04/22/s...ets-japan.html I also really appreciate the many genius designs of cat beds like various breads, dinosaurs, etc.