10's of thousands of animals have been tested for a predecessor to human Covid-19 though, for two years now, and nothing has been found, despite looking very hard and in ways that 'should have' found something had it been there. In all other known zoonotic crossovers, where we had the modern capability to test, the virus or a very close relative was quickly found.

Since we have exhaustively eliminated zoonotic origin, whatever is left must be the cause, even if it did not have the preponderance of circumstantial evidence that lab escape does.