Because those mutation events in a small number of people still require a longer time to become "stable" in the overall population of viruses.
Generally speaking, the more virus "generations" you have, the more likely you are to generate a successful variant. But then it takes time for that variant to achieve dynamic equilibrium in the greater population of viruses. For it to take over.
And the initial SARS-CoV-2 had so little diversity for so long, that we can say it likely had been stable before passing into humans, or there would have been more initial diversity in it compared to its closest viral relatives.
It is a picture overall consistent with a random crossover event. Not ruling out a lab leak (because that's quite difficult if not impossible to do). The absence of evidence is not evidence of absence.
But we have just as much evidence to say the virus came from aliens who planted it in humans as we do to say it came from a human lab that has no trace of the virus anywhere in it.