Why cancer is still not curable at a later stage?

Complicated answer. My best attempt to cover in 400 characters is this.Once cancer has spread through the whole body as it does in a late stage, the horse is out of the barn. There are too many microscopic cells to completely remove each one, and the cancer isn't in just one place to cut out. There are many excellent therapies which can dramatically reduce or delay recurrence in later stage cancers even if not cured.
Could kill patient. Simplified answer, not universally the case:goal is to kill all cancer cells.Some chemotherapy kills cancer cells because cancer grows faster than "normal" cells.Unfortunately, some "normal" cells do grow fairly quickly or are very active (hair follicles, surface of the gut, bone marrow), and they suffer. In advanced cancer, amount of chemo required to kill cells would kill patient.
Some are cured.. Testis cancer for sure - lance armstrong; but most cancers have multiple mutations, and no two are alike. Much of cancer is a regression to behaviors of cells normally expressed in embryogenesis, now out of control. Even with new findings, the goal is to make cancer a chronic, managemable disease like diabetes, as the tangled molecular messes are sorted out.