First, I wish you had a different screen name. Dummy is so negative and it often makes a person feel bad about him or herself. Besides, what you ask is not a dumb question.
The simple answer is that cancer was always present but people usually died at an early age from other causes such as smallpox, the flu, and measles. We live longer now so cancers often have time to develop. Also, we now know how to diagnose cancer. Years ago, doctors knew that people were sick but had no way to really study their patients. Finally, we live in a very polluted world. Many of the things that pollute our earth cause cancer in many people. Anti-smoking laws have done some to help take a dangerous cancer-producer out of out daily lives, but some people still smoke and some live with second hand smoke. And there are so many other things that cause cancer. We learn more every day.
Finally, cancer is not well understood. Different cancers have different causes and different treatments and we don't know that much about how to treat cancer. Scientists make progress every day, but what works for one type of cancer doesn't work for others. And some cancers are very hard to find because they do not make the person sick until they are really advanced and very hard to treat. And the person who answered before me is so right. Cancer causes so many other problems. It is expensive to treat, it causes grief in the patient and family, and makes hard decisions part of loife,