Cancer colonies self-sustain and self-replicate, causing local success, then kill the host without escaping it, causing colony failure and do not successfully become a new species of independent pathogen. Local optimization causes disaster at a larger scale, causing the system to collapse.

Nothing intelligent is happening. Zero-foresight self-amplifying systems that will inevitably crash out will simply amplify themselves to that point, then cease. Applications to areas outside of oncology are an exercise for the reader.