But cancer isn't a parasite. It's a flaw in an information system. We have several gates, oncogenes, that protect us against cancer forming. But then those genes take damage, then cancer can form.

This isn't a determinist "cancer will form or it won't" situation-- there's a lot of variance and luck. Furthermore many cancers form later in life, after people have already reproduced, so natural selection doesn't have that strong of a hold on it. If cancer worked the way you think it did, don't you think that cancer would have been naturally eradicated during our evolution?