Because most coffee is not tested for what is most likely to be in it. Ochratoxin A and aflatoxins. By-products of the moulds that grow on green coffee during transit and storage. Small amounts are common. Most roasters do not measure.
Green lots go to an independent, ISO 17025 accredited laboratory before roasting, and each report is published on the coffee's page as its batch clears. A lot that failed would be composted. Not re-tested. Not blended into a larger lot. Composted.