I don't understand the assumption that criminologists should somehow know how to commit the perfect crime.
I'm not sure any degree teaches that
Those who study criminology will not necessarily learn much about forensics, personally I did not. Criminology is mainly the study of crime and why it happens, criminal behaviour (the motivations for committing the crime etc.) and the penal system/application of law (punishment/rehabilitation, public responses to crimes, how/why laws are made, possible prevention of crime etc.), alongside other aspects.