For one they are aware they (as a population) get stomach cancer more often, so they actively screen for it, in the way most countries in the West screen for colon cancer.

Remember that some types of cancer, if caught early enough, are more akin to a chronic condition than a delayed death sentence. Once treated there are quarterly or bi-annual checkups and any suspicious tissue will be removed preventatively. You still have a much higher chance of stomach cancer popping up again vs. someone who never had it, but with the checkups it’ll be nipped in the bud before it grows into something severe.

That might sound offhand but that is how it is for (as far as I’m aware) bladder cancer and most gastrointestinal cancers. Cancer in the lungs, liver or brain usually is much much more dangerous because there’s little screening and they’re much more resistant to treatment.