Cancer not random lah, just human technology not able to solve it well.
Cancer at the core of the idea is basically saying your cells suddenly decide that they don't do what they usually do to help you survive and become something else.
There are so many types of cells so from your liver, to stomach, to kidney to even your private parts and all are different types of cells that serve different purpose.
If somehow you lucky, the cells become something else but don't attack your system, that is a benign growth. Which is those lumps that people find in weird places but don't really harm them. Most of your moles, that's a benign growth.
However, most times when this cells change, they become something that is harmful to your system.
Current tech only just using radioactive rays to blast all cells, including healthy ones, and hopefully blast the cancer cells within an area. No matter how advanced is not really that sophisticated as people might think. So that's why to survive cancer means you have strong lifeforce.