I think the problem is that cancer cells eat and produce waste pretty much like non-cancerous cells, so it's not easy to tell them apart. Cancer cells 'just' have the problem that they don't know when to stop dividing.

But, also, scientists are working hard to try to find specific chemicals that are produced by cancers. Is there a blood test for prostate cancer now? Maybe you'll find a test for some specific chemical produced by a cancer.

Cancer cells do produce waste, but that waste is no different from that of non-cancerous cells. It's difficult to test for cancer because you don't know whether the waste is coming from healthy cells or cancerous ones.