I think this is something intrinsic to animals. Working dogs are the same way: After some x amount of time, the dog is done working for the day. It doesn't matter what you do, the dog is done working. It's just something about work which engages the brain that animals can only handle so much of. I have no evidence to back this up, but I like to think this is a distant cousin of the neurophysical phenomenon of "overstimulation" where saying a word over and over causes it to lose meaning. It's believed this is due to a kind of neuronal over-use of some particular piece of the brain related to the meaning of the word.