I've hung out with a lot of pharma folks, and the business is really complicated. Most of the companies aim to both help people and to make a lot of money, and will choose projects based on some balance of those -- sometimes in dubiously ethical ways (e.g. tweaking formulations of existing products to extend the patent) and sometimes in basically fine ways (e.g. lots of the improvements in diabetes management which have helped a ton of people; at least the targeting is fine here though often not the pricing). Obviously chronic diseases that affect lots of rich people are a prime target to make money, so drug companies make lots of drugs for these, and they usually aren't curative because chronic diseases are hard to cure (but see e.g. Hep C which is now curable). There are also companies with rich investors who want to cure death, or at least cure particular diseases that they fear or have a genetic predisposition to (lots of unethical behavior from a certain now-defunct company that tried to do this).

For cancer in particular, pharmas don't (and mostly can't) just target a drug to chronically treat some cancer over the long term but not cure it. Instead they pick some target that's believed to contribute to development (/ metastasis / treatment resistance / whatever) in whatever cancer, and make a drug to interfere with it or to target an immune response to cells that make it. If it's stable, nontoxic, and looks potentially effective enough they'll take it to clinical trials. During clinical trials they'll find out whether it does nothing, gives you a few extra months, or has a chance at curing the disease. Usually the answer is that it does nothing or almost nothing, or isn't worth the side effects, and then the company wasted its time and money. Drugs with a chance to cure common types of cancer can be enormous successes -- see eg Herceptin.

Cancers are difficult diseases and it's rare to find something that reliably cures them. But drug companies aren't pulling their punches. Like they would never say "oh this drug clears breast cancer too reliably, we should make it less effective so that people will be more likely to die but also might take it for longer".