Source: https://en.wikipedia.org/wiki/2019%E2%80%9320_coronavirus_ou...
They know something that you don't. Which is that the test has both a high false negative and a high false positive rate. Which means that its effectiveness as a decision making tool is limited.
That said, the longer you wait to start quarantine, the longer you have to keep quarantine measures in place. But on the flip side, if extreme responses are too good, then you increase the risk that on the next serious threat, people will yawn and fail to comply.
Public health has a wicked problem here. The more effective it is, the less that people feel it is needed.
I think a lot of people would rather just let the virus spread.