Because, if there was a non-human made virus being studied in a lab which was human-infectious...then that virus would already be out there in the wild unless you'd made a specific effort to eradicate it like we did with smallpox.
"Lab-leak" is non-sensical in all forms which don't begin with "lab evolved/bioengineered" and while not completely implausible, the methodology for selective rapid-evolution of a virus requires the exact same conditions as are trivially recreated in the population and human-animal contact regime of greater China.
> "Lab-leak" is non-sensical in all forms which don't begin with "lab evolved/bioengineered"...
That's exactly the allegation circa March 2020 though: scientists collect various bat-borne coronaviruses (e.g. RaTG13) in Yunnan province around 2013, evolve one of them at the Wuhan Institute of Virology though gain-of-function research, and then due to lax safety protocols end up accidentally leaking an evolved virus in late 2019. For example, see this podcast from a year ago: https://www.youtube.com/watch?v=q5SRrsr-Iug
"Lab-leak" is non-sensical in all forms which don't begin with "lab evolved/bioengineered" and while not completely implausible, the methodology for selective rapid-evolution of a virus requires the exact same conditions as are trivially recreated in the population and human-animal contact regime of greater China.