Pro athletes are healthy, but they’re also typically very young, often in their early to mid 20s, at which age cancer is very rare in almost any population (less than 1% of cancers are seen in this age group). On top of that, most people are pro athletes only for a brief time before injuries or competition force them out, so there’s only a brief period when they could both have cancer and be a pro athlete.
In spite of that I can easily think of a fair number of athletes who had cancer. Lance Armstrong may be the most famous. I know baseball better than other sports, and just off the top of my head I can think of Jon Lester, Andres Galarraga, Dave Dravecky, Mike Lowell, Mark Loretta, Daryl Strawberry, Derek Lowe, and John Kruk, all of whom had cancer during their playing careers; I’m sure there are many more.
The question details say “They eat very nutritious diets, exercise constantly, and sweat on a daily basis. Some research suggestion that these type of activities help to reduce the cause of cancer“. I wish people would get this sort of crap out of their heads. It’s wishful thinking and victim-blaming. It’s like telling people with cancer that it’s their own fault for not eating better or “sweating on a daily basis”, whatever that’s supposed to mean. People get cancer for many, many reasons, and though it’s true exercise and healthy diet can somewhat reduce cancer risk it’s a much more minor factor than people seem to think.
Cancer isn’t retribution for not eating enough spinach. It’s a disease that anyone can get.