The incentive is to continuously charge these people for the rest of their life. They make more money if you stay sick, so they can keep treating you.
If you are cured, then the flow of money stops until you get sick again.
If, however, after a lot of work, a cure seems elusive, it makes sense to focus on treatment. Both for the sinister reason that people pay more over a lifetime for treatments, but also for the less sinister reason that it's usually much easier to develop treatments than cures, and having a treatment and not having a cure is strictly better than not having a treatment and also not having a cure.
What is stopping them from discovering a cure, patenting it, and then never manufacturing it?
Also, this presupposes that the companies are actively trying to do evil. In reality, most people would rather have cancer cured than not cured. The profit motive can bend these incentives, but I think the caricature of a company full of amoral sociopaths making decisions to maximize human suffering are kind of... unrealistic.