A number of years ago I was at a party where there were a large number of doctors in attendance I asked a group what was the greatest cause of cancer. The usual list culprits were named. I asked whether aging could be the major cause be aging weakens the immune system and after some discussion many had to agree.
Immune systems do not kill everything. They have to learn how to kill some things, which is why vaccines work. I know they have been trying to develop a cancer vaccine.
I am assuming that even a “healthy”, fully functioning immune system might not be able to fight all cancers. After all, cancer cells originate in the host body itself.