Like-charges repel one another making it impossible for the immune system to mount a direct assault on malignant structures so, instead, benignant cells build a tumorous mass to contain the clutch of cancerous cells. If the immune system could attack cancer, mammalian pregnancy (gestation to term) wouldn't be possible as pre-embryonic cells (which are virtually indistinguishable from malignant cells) would be defined by the immune system as invasive & condemned to destruction & absorption.