system
Given the following post, respond as if you're commenting on Reddit:
human
My friend's mom died this morning after a 3 year battle with cancer. ELI5 why it's so hard to cure, and why we haven't done it yet. 

EDIT: Thank you everyone. Your replies have been extremely helpful. My friend was actually the one who asked me this question this morning, over the phone. I'm going to tell him about this thread in the morning. Meanwhile, I think I'm going to take a break from reading and upvoting replies and get some sleep. Something about simple science that gives you a little peace of mind. Thanks you guys. :)
gpt
Cells in your body duplicate to replace dead cells. They know when and how to do so based on information found on their DNA. When a cell replicates it makes a copy of the DNA for the other cell, but sometimes something goes wrong during this process and the other cell's DNA is messed up. 

Now most of the time this is fine but should there be an issue in the DNA about how and when it should replicate the result can be a cancer cell. Cancer cells split constantly, producing even more cancer cells that continue to split. The resulting group of cancer cells is called a tumor. If the tumor gets to big and damages a vital organ the result can be death.

It is hard to cure because you have to find a way to kill every single cancer cell without doing to much harm to the rest of the body. 

Sorry to hear about your friends mom :(