Sounds like the environment of humans is the explanation. If you were to put a whale in a human city, eating and working there, maybe the whale would have the same rate of cancerous cells
> A 2015 study, the San Diego Zoo, surveyed results from 36 different mammalian species, ranging in size from the 51-gram striped grass mouse to the 4,800-kilogram elephant, nearly 100,000 times larger. The study found no relationship between body size and cancer incidence, offering empirical support for Peto's initial observation.[8]