The treatment you don't need might be zero risk, but it might also have quite a bit of it. And some tests carry risks too: See how different countries have very different protocols. The US system, which is colonoscopy first, and where polypectomies will be done at the same time, can end up with bowel perforations when the polyp was not even cancer. So the diferente treatment protocol is not just about costs, but about balancing different risks.
The unnecessary test, leading to an unnecessary intervention, which gets you unnecessary long term issues is a real risk in many cases. It's not just a matter of doing a bonus blood test here or there.
To take it to the extreme:
If there were a risk free button that you could click to see a full breakdown of all illnesses no more how early stage, would you click the button?
It would include a report of cells that may eventually turn into cancer (or may not).
It would include things like polyps, and every other diagnosable condition.
Do you avoid pressing the button? Why?