odds of it happening eventually, go up the longer you live.
Despite this DNA replication being highly controlled and very accurate, the sheer number of times it is performed in the lifespan of a person (estimated to be 10,000 trillion times!) means the introduction of a significant number of errors into the DNA of some of our cells from this fundamental process is inevitable. source
I wonder if it just takes longer to become a problem? I would expect (and someone correct me if I’m wrong) that a tumor likely grows at roughly the same rate regardless of body size. A 3cm diameter tumor likely just isn’t as big of a deal when you mass 100,000kg.