Most whales never come close enough to land to see humans who aren't in boats. Divers are pretty rare.
So they might think of humans as boat brains.
Boats have brains, but they're not very well attached. When a boat brain falls out of the boat, usually the brain dies. Fortunately, boats have more than one brain and can survive losing one.
Why? It's not like there are suddenly no humans at sea. Small fishing vessels may have gone down in number but in the meantime leisure vessels and the sheer number of humans have gone up (it's massive). Humans are ⅓ of all land mammal biomass. Whale scientists are probably more puzzled by the lack of species diversity among land mammals that visit the sea.