system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5:How come some people smoke their entire life and live to be 90-95..while some people never smoke and can get lung cancer and die?
gpt
Cancer is a game of chance.  It's not just simply damage to your DNA, it's damage to DNA in key places.  There are proteins that check for damage to DNA and repair it, so your damaged DNA either needs to be too bad for that protein to fix, or the genes responsible for building that protein need to be damaged.  There are proteins that detect irreparably damaged cells and force them to self-destruct so they can't copy that damaged DNA, so the gene for that protein has to be destroyed.  There are specialized immune cells that detect damaged, cancerous cells that aren't self-destructing, and destroy them, so the gene that signals to outside cells that it needs to be destroyed has to fail.  And of course, you have to damage DNA such that the cell reproduces when it isn't supposed to.

That's a lot that has to go very wrong for cancer to happen.  Every time you smoke, you're rolling the dice.  You may damage a lot of DNA in a lot of cells, but you might be damaging the DNA that controls the growth of skin cells, but the DNA is in lung cells.  So nothing happens.  Or you might get rapid cell division, causing that cell to be cancerous, but it self-destructs before it can cause problems, etc.  Some people just get very lucky, the same way you might roll a die fifty times and never roll a 1.  Or you might roll the die three times and roll a 1.

There are some cellular mechanisms that biologists don't fully understand that prevent and control cancerous cells in our bodies.  Some people for some reason are naturally resistant to cancer for those not-understood reasons - their cancer detecting proteins work better, or their cancer-eating immune cells work better.  Or there are other factors, other risks that aren't directly related to the smoking that cause one person to get cancer sooner but that don't affect another person.

There are a lot of things that increase your risk of cancer.  Really, when someone says something "causes cancer" what it means is that those things make your risk of getting it go from a normal low background risk to "You are almost certainly going to get cancer".  But it's always risk, always different factors adding up that make your likelihood increase.