It was engineered. They have admitted that. Coronavirus was manipulated in the Wuhan Institute of Virology to become more infectious. The contention is around the term "gain of function." Fauci changed the definition some years ago to skirt the funding bans imposed by Obama. His new definition requires intent to make a virus more communicable or virulent for humans. Since the WIV didn't intend to make it more communicable or virulent for humans, Fauci maintains it is not "gain of function" research.
I don't see why this is so hard for some people to accept. Chinese laboratory standards are among the worst in the world. Article after article after article details their abysmal safety standards. Why on Earth should we believe that this specific lab were somehow better? Of course it wasn't. Of course some unqualified technician made a mistake. This was always the concern about gain of function research, which was why Obama banned it.