How come surgical removal of a lung not stop lung cancer?

When already spread: Resection of a portion of lung will cure a lung cancer when, and only when, there is not a single cell from the original tumor capable of starting a colony anywhere else in the body. Unfortunately, in a majority of non-oat-cell cancers and almost all oat cell cancers, at least one cell has made it via a lymphatic or blood vessel to a remote site. There's no being sure and surgery's often worth it.

See below: Surgical removal of the tumor may only ensure removal of gross visible disease but in most cases of lung cancer the cells from the tumor likely have already migrated to distant sites and may be potential sites that cancer may develop. Most patients with lung cancer may benefit from chemotherapy after surgery to decrease likelihood of the cancer coming back.