Two reasons. False positives and the medical interventions that result aren't without risk. Some cancers are just a death sentence. They start out virulent an

Two reasons. False positives and the medical interventions that result aren't without risk. Some cancers are just a death sentence. They start out virulent and spread quickly. And are immune to treatment. Detecting them early buys you nothing. Flip side is some cancers progress slowly. Screening tends to catch the slow indolent cancers and miss the nasty ones. But because they progress slowly catching them early may provide not much benefit.