There are two reasons that I’ve heard of (I worked in imaging & was involved with the tumor board, & research). One is that different cancers grow at different rates, many driven by hormones. As we age, our hormone levels decrease, & things typically grow more slowly. This often means that although you have a cancer, it isn’t likely to be the cause of death.
The other reason is again age related, as you know, cancer treatment is extremely hard on our body/organs/blood, and the likelihood of the treatment causing death, outweighs the likelihood of it curing cancer, &/or improving quality of life.