This is actually an interesting open question, known as Peto's paradox [1]. The "hypertumors" hypothesis is especially fun - a tumor has to get really big to af

This is actually an interesting open question, known as Peto's paradox [1]. The "hypertumors" hypothesis is especially fun - a tumor has to get really big to affect something the size of a whale, and a tumor that big needs a fair amount of internal structure to support it, and tumor cells have a demonstrated tendency to misbehave already, so the tumors get cancer of their own and die before they can get big enough to affect the host. [1] https://en.wikipedia.org/wiki/Peto%27s_paradox https://en.wikipedia.org/wiki/Peto%27s_paradox