Process of elimination then if no tests

I keep reading over and over there tests aren’t available for covid-19 - I’m not getting why doctors aren’t just using a decision tree:

1. upper respiratory illness with fevers chills dry cough etc
2. does it test positive for flu a or flu b?
3. is it pneumonia?

No to the above? Then it’s covid. Presumably the flu and pneumonia are easy to rule out - I don’t get why those cases aren’t tested, then treated then as covid, and quarantined with a standard care protocol.