Why is Cancer So Hard to Cure?

Cancer, unfortunately, is heterogeneous, simply meaning it can be controlled by many cells and has the potential to spread to many regions of the body. As we get older more and more mutations begin to occur and our repair systems begin to break down. Now, for some terminology I'd like to share with everyone. In our bodies, we have oncogenes which are normal genes that have been mutated into cancerous genes, as well as tumor suppressors which, as shown through the name, are physically genes that protect a the body from forming more oncogenes. I'm going to leave off here for now, at a very basic level and hope to give you more  insight in the near future.