People don't claim they made a virus from nothing, it's more to do with genetically modifying exist virus to change them into something more lethal. The same thing happens on the end of the scale to make medicine more effective.
One theory is that it could have been the virus visnu (which is very simular to HIV)genetically modified with Human T-cell Leukemia Virus, which affects white blood cells.
The ethical reasons are something to debate, but I wouldn't say that it is physically impossible to do this sort of shit.
The facts are that AIDs appeared in the late 70s and there is still no known whereabouts of the source. The official account is that it came from monkeys, but this is still a theory, which is yet to be confirmed after many years of research.
The reason a lot of people debunk the idea that is was made in a lab is becuase of contraversial ideas behind it. Scientifically speaking, it's still one possible place that AIDs came from according to a lot of scientists.