You're missing that in a free market, companies will try to compete with one another. Strictly focusing on treatment is far too risky an endevour if another company is working on a cure.

It's all risk-reward. A company with a cure is certain customers will come to them, as anyone would pick a cure over treatment. Additionally, they can ask for a high price: it's your only option to be cured after all! So therefore, a cure is low-risk high-reward. Exactly what investors like. It's also why there is a lot of research going into curing cancer, which disproves your hypothesis.

If company A does treatments but company B cures, then B stands to make a lot of money and A sees their revenue stream cut off overnight. This means that in order to remain competitive, A must also research a cure. It's in their financial interest, as it is the only guaranteed way to keep making money.

The reason we don't have an amazing cure for all cancers yet is because cancer is difficult to cure, not because there isn't any funding. We've already cured a couple types of cancer, but no one cancer is exactly the same.