Because like you said it's basic work. It's a base. I think it's executed very well, but there is nothing really special about it yet. Needs a highlight, some flavor.
Having little joyful jumps or bobs could be the next stage of dog happiness. Then on it's back doing the belly scratch offering.
I can't imagine that a game only about petting would keep players interested for long. Important additional mechanic: Overfeeding, getting dog fat :]