The neck muscles (platysma) do not have a fixed attachment like other muscles that attach to ligaments and bone. The platysma muscles are free-floating in the neck and actually are pulled forward by their own contraction.
In facelift surgery, I pull the back portion of the muscle close to the ear again. You can see this in yourself in the mirror. Just pull on the neck at the angle of the jaw towards the ear and see the neck tighten. Once you do this, you will see why most non-surgical treatments for neck sagging don't work if the muscle position is not addressed.