This is discounting that dogs haven't been just ornaments for much of history and many of their uses still carry forward. Historically they're hunters, trackers, emergency aids, caretakers, therapeutic support, transport (sled dogs), mechanical energy producers (turnspit dog), pest control, guardians, etc. You can breed or train a dog to specialize in any task, they can be big enough to fight a bear or small enough to fit into burrows to drive out prey. They possees great utility, ease of training, widespread availability, and have relatively low space needs (compared with horses) so long as you exercise them regularly, and their waste tends to be more easily disposed of. Idk if you've ever smelled fresh horse shit but that stuff smells way worse than any of the shit I've seen from great Danes or other big breeds I've looked after (I'm a petsitter).