The answer is that the distribution of cells bearing the mutation(s) is spatially restricted, local to the organ that hosts the tumor. For example, a squamous cell carcinoma results from mutations in dermal skin cells, causing cancerous growth. The mutated cells are limited to the tumor itself. Hence, removal of the tumor will remove all of the mutated cells. Some cancers can become malignant and will spread to other organs. The mutated cells are defined by the original parent tumor and the sites of metastasis where those cells have migrated, elsewhere in the body.

Some cancers come back because whatever treatment - surgery, chemo, immunotherapy - has failed to eradicate all of the cells bearing the cancer-causing mutations.