Culture negative pneumonia is a relatively common phenomena. It's also quite difficult to get adequate sputum cultures, but in this case where n=60 I'm skeptical that was the issue. There could be a lot of potential causes for this, but COVID is low on the differential.

I want to say that spillover events, or a virus jumping from X host to humans, happens all the time. COVID became a pandemic because human to human transmission was possible from genetic mutation. Usually with spillover events humans are a dead end host and the person infected will be an n=1.