It's usually becaise they have lower metabolism so their cells don't suffer as much damage as ours, and they probably have a lot of copies of genes that prevent cancer.
I wonder if whales survive to an equivalent of human old age (where we can be pretty fragile and still survive for decades).
They do and this is well-documented. They're one of the few (only?) other species where we can directly see the role matriarchs (grandmothers, either past their reproductive ages or even undergoing menopause) have on the development of the youth, and it's been used to explore the role of living post-reproductive ancestors might have on natural selection (which was previously not thought to be affected by it).
Maternal grandmothers have even been found to ~spontaneously regain their milk to help nurse infant whales, iirc.