Once they were in a car, in your speculation, they could have been taken anywhere. Potentially even to a murder/disposal site where they would've been much more hard to find (or, to a place that was much easier to find, if display was what the offender had in mind). So why do you think, to that particular place where they were killed and ultimately found? The abduction site where the victim is taken is almost always more random (being wrapped up in opportunity) than the murder or disposal site, which can be quite planned out.
I get it. It's all speculation.
The actual murder site could have been prepared in advance. It may have simply been familiar to the murderer. The killer may not have had any desire to cross a creek to commit the crime. Maybe simply access to the site from that cemetery provided ease of escape, unseen. Possibly he wanted the girls to be found, eventually.
In light of the crime being unsolved, and for me personally, in terms of the statement that the youtube videos all got it wrong, I began to speculate on what alternative scenarios may have transpired. Speculate, being the key word.
For example, I've heard the question, 'why did nobody notice the wet pants' on the killer? Well, maybe because he was gone from the murder scene and never even crossed that creek. Maybe he never backtracked to the trails after the crime.
Alternative theories, for me, often revolve around two premises.
One, the girls were targeted in some way, one or the other or both of them, and the killer was aware of their visit to the bridge that day, or at minimum suspected such. The murder being revenge, or some other reason, say, to cover up other nefarious crimes of the killer.
Two, the girls were victims of a killer who had been laying in wait at that bridge site, for a victim, any victim, and the girls were at the wrong place, at the wrong time.
In either case, it seems to me, the killer(s) was/were somewhat familiar with the territory there.