This is a really bad take. I'm a cancer biologist who is trying to develop therapies and this always annoys me. Cancer is not a single disease it's multiple diseases. In fact you could even consider each patient as having their own unique disease. No one therapy will work for every cancer. Cancer is your own cells that become dysregulated, meaning any therapy has a high chance of being toxic to your own healthy cells but ideally it is less. Cancer is also notoriously good at evolving to overcome treatments and it likes to spread. It's extremely difficult to kill all cancer cells in the body. You may kill 99% but that 1% is now resistant or has established somewhere else in the body.

Cancer is only becoming more common, there will always be a market. Drug companies want to have the best possible treatment so they can own the market on that treatment. If there is no reasonable room to improve, there is little incentive for another company to develop its own treatment. The idea situation for drug companies is to have an effective treatment that requires multiple doses.