http://www.telegraph.co.uk/news/2017/03/23/two-thirds-cancers-unavoidable-even-live-healthy-life-study/

Other sources show the same story, the fundamental explanation is our genes mess up when they are replaced.

"The results showed that it generally took two or more critical gene mutations to trigger cancer.

In some cancer types, such as those affecting the prostate, brain and bone, more than 95% of the harmful mutations were due to random DNA copying errors.

Copying mistakes were linked to 77 per cent of pancreatic cancers but only 35 per cent of lung cancers, which were mostly triggered by smoking and other environmental factors.

Overall, 66 per cent of cancer mutations resulted from copying errors, 29 per cent from lifestyle or environmental factors, and just 5 per cent from inherited faulty genes."