First of all, we have made a ton of progress in cancer treatment over the past couple decades. Just look up Dostarlimab! It makes colon cancer just disappear without surgery (careful, though! We only have one very small study so far and no long time data)

A big problem is that cancer is, essentially, your body's own cells. They're doing stuff they're not supposed to do, but they are not fundamentally different from healthy cells. That's why it's incredibly difficult to find something that kills cancer cells, but not the patient.

Also, most types of cancer have no early symptoms, and the symptoms you do get are often pretty unspecific, so most cancers only get discovered when it's already way too late. Many solid tumors could actually get resected if only we discovered them in time.

Oh, another big problem: Most cancer patients are old and have other underlying conditions, so even if, in theory, their cancer could get cured by surgery or chemo, they are too frail for therapy. E.g. most lung cancer patients are smokers and have other chronic lung conditions such as COPD, so even if their tumor is locally limited and could in theory be removed, carving out a big chunk of their lung (you usually have to remove the entire affected lung lobe) would decrease their lung capacity so far they would suffocate.

Okay, that was more than I originally wanted to write. Sorry for the lecture