Are they tumors, or conditions such as Lymphocystis?
Lympho is a viral disease that results in small growths. There is no treatment (because it's caused by a virus). It's usually self-limiting though, which means that given time, the fish's own immune system can often fight it off.
Also, not all tumors are cancerous. Many are benign. (This is true of people, too.) If a tumor is benign, it can look bad but won't cause significant health effects for the fish (unless it grows someplace that vital, and interferes with the fish's ability to breathe, eat, etc.)
Some tumors also have a genetic component, which can be passed down from generation to generation. This may make some "families" of fish more susceptible to tumors.
And there are probably many environmental factors involved, too. For example, use of certain chemicals can make organisms more susceptible to the development of certain types of tumors and/or cancers.