Please explain why is cancer classified as a disease instead of a genetic defect?

That is an AWESOME..: ...Question! cancer is, after all, caused by changes (mutations) in certain genes that control the way cells behave. These changes usually develop during our life although sometimes they are inherited from our parents. The role of genetics in oncology (and probably other specialties) grows in importance with each passing year so the boundary between those 2 things can be hard to draw.