Staying Cancer free has a cost, in terms of DNA repair (proteins) and their optimization, immune system and probably more elements. If a species reaches a point

Staying Cancer free has a cost, in terms of DNA repair (proteins) and their optimization, immune system and probably more elements. If a species reaches a point at which it can reproduce without incurring too many germ line DNA errors, there is no need to optimize further. I.e., if there was enough evolutionary pressure, our cancer incidence would also still be able to go down. However, the current incidence is not affecting natural selection at a significant magnitude. Same for mice and whales I would guess.