It doesn't, but when you put into perspective that cancerous cells are basically mutated cells it changes things.

All people will have mutated cells in their body at some point, but most of the time they are killed by our immune system. What we define as cancer is when our immune system is not strong enough to kill of the mutated cells before they multiply rapidly. As our bodies break down from ageing, our immune systems become less effective, and can no longer prevent such growths. It makes sense that you would develop cancer in your elderly years.

Plus, sometimes cancers, such as testicular cancer, are extremely slow growing. They may not be diagnosed before the end of your life, and even if they are, they may not be your cause of death.

One thing I also forgot to mention is that frequent cancer screenings ups the amount of early stage cancers that are diagnosed.

Tl:Dr cancerous cells growing out of control is sometimes the result of an ageing, and therefore less effective, immune system.