Well, it's because it's not "cancer cells" as something different then the host, it's just "here's some cells that, for whatever reason, decided to start growing and multiplying indefinitely, rather then being restricted by what their programming says". Comparatively, cancer is far "less threat" then most diseases, except for the fact that your immune system treats it like part of your body. Thanks to that "leg up", it can do a lot of damage. If your cancer got into another person, their immune system would treat it as foreign (modulo transplant factors and such), and attack and destroy it.