A pet dog isn't physically dependent on the body of someone else to remain alive."
Yes, it is.
It is entirely dependent on humans to provide it food, just like human children, or other human adults for that matter.
Even strays or (semi) feral populations of dogs are totally dependent on humans for food, whether directly given or garbage pickings. This is why you don't see them establishing independent populations in the wild.
Our children aren't considered adults till the age of 18 for the same reasons as well, unable to survive on their own. Heck, the latest generation are still dependent on them a decade after.