The incentives are not alligned properly. Cancer patients don't pay as good as advertisers. It would help if everybody would have an expensive cancer insurance. But I guess most people don't think about cancer until it's too late. It's more fun to buy stuff after clicking on ads.

Edit: the incentive of an insurance doesn't work either :( . It should be possible to bet money on getting cancer. Everybody who bets against me gets my money in the case that I don't get cancer.