It's simple. Cats aren't fetching per se. They're playing Chase the Ball, and they bring the ball back so their human can throw it again.
Whether or not we consider the basal instinct as retrieval vs some other similar instinct that can be easily used to motivate the retrieval behavior [1], I’d wager most dog owners that have trained (or attempted at least!) would agree that some dogs are just more likely to become good retrievers which to me suggests that there’s more than just a basic pleasure stimulus response.
Similar dog behaviors based on breed that I would argue at least point towards instinctual behavior being at least a partial explanation for behavior are herding and live stock protection breeds. Take a quick watch of any of the sheep herding competitions or other fun related videos [2] to see what I mean.
However, at the end of the day I also think that most “instincts” are more of a shorthand/stereotype for some set of abstract probabilities that may or may not result in a dog being good or bad at any specifically desired behavior.
[1] https://www.wildfowlmag.com/editorial/tips_strategies_wf_ins... [2] https://www.youtube.com/watch?v=D2FX9rviEhw
Because we fundamentally disagree on the intent. I don't agree that a cat is engaging in parasitic exploitation of the human toy-thrower, and I don't agree that a dog is toiling in servitude to the thankless human who keeps throwing away the retrieved item. They both love chasing things, they both understand that to chase the thing again they need to return it to the human, and they both appreciate the positive reinforcement, bonding, petting, and attention when they do so.
A dog however simply does not do things on its own much, dogs are maybe even more social than humans and live in packs (dog owner is de facto pack leader). Play is more social for them. Take out peer/leader to play with and half the excitement is gone. Dogs generally would not consider mechanical ball dispenser equally exciting of a partner