Simply because the inventor of the test makes it clear it was not designed to identify a disease.
You are mixing up the ability to identify a virus with identifying a sickness.
PCR identifies very well the presence of virus.
Presence does not indicate the disease has been conferred, symptoms do that. That is Mullis’ point.
We see this in the huge number of asymptomatic cases, people who test positive but don’t get sick and people who have antibodies meaning they were infected but never knew they were sick.
It’s not an unidentified virus. That was the phrase you used that didn’t make sense.