[mrosett]

They have immense adaptive pressure at the cellular level. The evolutionary process plays out within each patient. Aberrant cells that don’t have some sort of evasion mechanism will be identified and destroyed by the immune system.

[geysersam]

I've heard a hypothesis that cancer behavior is not primarily evolved in the patient, but instead it is an ancient mechanism from the time of transition from single cell to multi cell organism.

A human cell can never survive on its own. But there might have been a time when cells could have benefited from "pulling the escape hatch" and taking their chances instead of continuing cooperation within a damaged early multicellular organism.

[dredmorbius]

I'd love to see a source on that if you have one handy.