Its really not as complicated as it sounds, it is riskier to move a deceased victim great distances, for fear of discovery, usually after the murder they want to get rid of the victim as quickly and quietly as possible (remember murder is usually secondary component to sexual assault which is overwhelmingly the driving factor in these crimes) therfore its a much smaller risk to move a living (probably compliant out of fear or con) to an area where they can not be seen committing the actual murder, in many cases, the victims are killed where they are found, in most the killer then hides the body and leaves quickly once they are cetian the victim is deceased. In a study done by the State of Washington Attorney Genrals office they found that in the majority of these cases, there was a spaital ralationship of between 1-199 feet between the actual murder scene and the spot where the victims body was recovered. This can also be due to animal activity and or post mortem movement by the offender himself (they often return)
The distance between the abduction site and recovery site is often greater than 5 miles, regardless of criminal experience remember the mean age of the offender at the time of abduction is around 27 (27.8 ) yo and they usually have a pretty diverse, and in many cases, extensive criminal background by the time they work up to abducting a victim.
The Key figures to keep in mind with these cases,
1) In Most child abduction murdes the victim is deceased within 1- 3 hours
2) In most Child abduction murders the victim isnt even been discovered/reported as missing for 2 hours after they have already been abducted ,
therefore in many cases, the victim is already deceased, before it has been discovered they were even missing
Thats why its so important to move fast in these cases,