"Wby do vegans think they can't get cancer?"
The reason why SOME vegans and also SOME think that they can't get cancer is that for SOME types of cancer are reduced. However there's an equal offset, that the risks for some other types there's a slight increase for the risk for some other types. With that said, some try to use what they've bought into, what SOME of those who are SUPPOSEDLY medical professionals has claimed, or otherwise tried to lay claim as fact. Those though that has bought into those who are vegan or those who specifically follow the strict vegetarian diet, which is the diet that vegans follows, is based on belief, and not scientific facts, but put forth by a few doctors, and some in the field of homeopathy, and those who accept what those two groups says, usually has at some level a high gullibility factor.
Yet there's one vegan, who is a regular in this section, who will quickly dispute such claims, as they're a cancer survivor. I have a great deal of respect for them, and their honesty. So not all vegans claim or believe that they're somehow immune to not only cancer, but also any of the other litany of claims of immunity to specific types of diseases. So any generalized sweeping broad sweeping statements about vegans, vegetarians, or any of the other dietary types, is a poor choice of either wording, or believe that all can be pigeon holed as being all the same.