If all 300 million Americans get blood tested each year, that is 1 and a half MILLION people who are falsely told they have CANCER. One and a half MILLION people whose life gets turned upside down, WRONGLY, due to a blood test -- at least, until they take a second one. Or a third one. Or maybe they get unlucky never learn they don't have cancer, and mess up their life treating a disease they don't have.
Now, OBVIOUSLY not all 300 million Americans are going to get tested each year. But you are completely ignoring the ethical concerns surrounding telling literally .5% of those who don't have cancer that they have cancer, and the non-insignificant proportion of people who will never learn they don't have cancer (we NEED to consider even the "miniscule" probabilities at this scale), and that makes you seem like somebody who "think[s] they know better".