The reason one gets cancerous tumors is the body fails to recognize cancer cells as abnormal. Cancer cells are created all the time in our bodies. They are our own cells which are defective. They tend to grow too fast. Most of us have the ability to recognize the abnormal cells and destroy them before a cancer tumor forms.
Therapies such as chemo & radiation can kill the tumor(s) but cancer will reappear exactly for same reason as it appeared in the first case. Especially true of stage-4 cancer where cancer has spread to many places.
I’m a medical doctor.