Also, exercise reduces colon cancer risk by 40-50% and breast cancer risk by 30-40%. Then there are other risk factors such as poor diet, insufficient vitamin D, etc. Overall it seems highly unlikely that cancer is mostly "bad luck".
50% of what though? If you have 1 in a million chance, increasing that to 1.5 in a million isn't a big change. If "bad luck" afflicted 1 in 100,000 then then bad luck would still have a much bigger effect, despite lifestyle changes clearly being significant.
Do these percentages take into account causation vs correlation? Apparently (surprisingly), it's still very difficult to prove causation without a laboratory experiment.
Perhaps people who are likely to exercise have a certain genetic disposition to begin with.
(These numbers totally made up)