Cancer is fucking evil. Sorry for all your losses and struggles. It killed my wonderful grandad over 3 months in such a horrific way.
Everytime your cells divide, you accumulate mutations in your DNA. Fortunately, the vast majority of DNA doesn't code for any proteins, so you're fine. But as you get older and accumulate more and more mutations, eventually these will fall on a number of genes called onco-genes which code for proteins that might make your cell divide forever, become immortal, avoid immune system recognition and gain the ability to create their own blood vessels.
It takes a very long time for all this to happen, but that's why, if we were to live forever we would all develop cancer eventually and why it's so prevalent once you get to a certain age.
Younger people who get it are extremely, extremely unlucky, or tend to be predisposed to it.
Chemo doesn't work well on solid tumours because it acts on dividing cells, targeting the cell cycle, and the majority of cells within solid tumours are not dividing. Once you get metastasis to another organ it is also practically impossible to cure the cancer and it can only be slowed.
That said, 50% of cancer patients now live for over 10 years after diagnosis, so things are improving! We need to focus more on screening and early treatment and then we would see an increase in survival.
Praying this evil horrendous disease can be erased one day.