I've often wondered about that. I have a 55 gallon tank, and a few of the fish are quite old. We have a goldfish named Brutus that we have had for almost 5 years. He's huge, and seems to hold a grudge. Our fish get fed in the morning and night (they are big and require a lot of food). If you forget to feed them in the morning, Brutus will wait until you open the lid and get close to the tank then tail slap the water to get you wet. He has done that to me many times, and has never done it unless I forgot to feed him.
So, I think they underestimate fish. I guess humans like to do that with animals, though. But, we learn all the time that various animals function on a higher level than we once believed- just look at the intelligence of dolphins.