Cancer usually refers to a cell mutation that is harmful to the organism. I'm sure DNA mutations happen all the time in individuals, but in most cases they don'

Cancer usually refers to a cell mutation that is harmful to the organism. I'm sure DNA mutations happen all the time in individuals, but in most cases they don't provide the utility to increase "fitness" relative to the environment which would increase the individuals quantity of reproduction/offspring, thus spreading the gene. If people were still able to reproduce at older ages, I'm sure we could evolve to be more resistant to cancer; since cancer will most likely affect you later in life, it has no impact to how much you reproduce.