I confess to not being a biologist and to also thinking that the lab theory is certainly possible.
However, if we’re ruling out (or at least making very very very unlikely) that it was engineered to be human infecting in the lab, then what happened is that, in a chance encounter between a human and a non-human infecting coronavirus, the virus mutated and was then able to infect a human.
Given that there have to be some safety protocols in the lab and not that many people there, whereas outside there are lots of animals, people and not many safety protocols, isn’t it a pretty conservative conclusion that there are indeed hundreds of times (at least) more interactions between humans and non-human coronaviruses outside the lab than in?