Well, if the immune system was able to kill cancer cells, there would not be a problem, would there?
Cells can self-repair to some degree and the immune system helps when they can’t, but its obviously not sure-fire.
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.