First, correlation isn't causation. Just because there are two 50%s doesn't mean they're the SAME 50%.

As for your question, oncologists can get cancer and even die of it, but that doesn't keep them from helping patients with cancer. There's theory and knowledge and then there's failure to do what you know you should do. There are marriage counselors out there with failed marriages. Doesn't mean they're not helpful to the couples they see.