I see this objection a lot. As someone with quite a bit of knowledge about the pharmaceutical industry, medicine, and basic research in biology, let me try to explain the problem: creating a cure is insanely difficult. Why?
1. It usually requires permanently altering cellular anatomy or physiology/metabolism, and homeostasis won't let you.
2. Many diseases have genetic components, which would require altering DNA to cure.
3. We don't have the technology to carry out #1 and 2.
In a disease state, the body's homeostasis has diverged from a "normal" state. Homeostasis is a robust process, meaning that it can take a lot to change it; usually it occurs slowly over a long period of time. Taking a pill that temporarily alters that homeostasis doesn't reset it to normal. Think freshman chemistry: equilibrium and Le Chatelier's principle. You changed the equilibrium, and the disease state homeostasis fights to go back to what it was.
As for genetic components, cystic fibrosis should be the easiest disease in the world to cure: it's caused by having two copies of a bad allele for a potassium channel that result in misfolded proteins. Insert at least one good copy into the genome and voila! A cure! Yet nobody has ever demonstrated success with gene therapy in humans. Which leads us to the third point...
We just can't figure out how to get gene therapy to work well. We also can't figure out how to permanently move a diseased homeostatic process (e.g., insulin resistance) to a normal state.
It's not that the pharma companies don't want to. Patient compliance with medication is horrible. If you tell a patient to their face that they will only live 3 more years if they don't take this pill every day, versus 10 years if they do, the average patient will only take the pill about 180 days of the year. So, if a drug company could sell a cure at the same cost as a lifetime of one-a-day pills (which they could), then they would absolutely do so. It's guaranteed money, like a magazine subscription versus buying a copy off the rack whenever.
Besides, many scientists at academic and government research institutions would rather find cures for diseases, yet are unable to. Unless we pull out the tin foil hats and speculate that they, too, are on the payroll of the pharma companies, it should be clearer that there are other reasons cures don't happen.