Acoustic imaging is a very interesting topic as the way we determine locations in multiple dimensions is based on observation (it affects how our brain processes what we hear, though we can shut it off and still locate sounds perfectly well but can be more easily fooled), frequency, amplitude and phase (the relative phasing of the frequency components in a signal). I did a lot of work in this area and it is amazing how we can fool our hearing. Positional information is also related to the shape of our ears and how our brain learns how to position sounds (experiments that pinned peoples ears back caused them to lack the ability to locate sound, which they then learned and then had to relearn when their ears were allowed back into their normal shape).
All in all it is a very complex subject and is related to data and the way our brain processes data. A lot of the current processed audio system use phase as well frequency and directional control to create the effect of moving the location of a sound. Dolby ATMOS does exactly this by allowing you to map audio objects anywhere in 3 dimensional space by using the amplitude, frequency and phase of the signal you are trying to locate in 3 dimensional space. As you point out stereo was one of the first systems that attempted to locate sounds in a 2 dimensional space. Surround sound does this in a plane in which you are placed and ATMOS tries to create a 3 dimensional space in which a sound is located.
Soundbars use some of this to create the illusion that you have widely separated sources from a small system where the actual transducers are only locate 36" to 48" apart. It has a lot of work to do and in general they depend a lot on location and adjacent reflective surfaces i.e. environment. Loudspeakers interact with their surroundings and this is what causes the problems with them functioning when trying to fool your ears into locating sound sources in space. Headphones have complete control of the environment and can more easily fool your brain into convincing you that a sounds is located in a particular point in space. I remember early tests with open backed headphones where the test was a person walking around you and whispering in your ear. It made you jump and you had no problem locating them in a 2 dimensional flat plane. Adding a third dimension is very hard with headphones however.
Anyway, after all this pontificating, I was just trying to point out that soundbars if correctly placed in an ideal environment can possibly fool you into thinking you are sitting in the middle of a sound stage, but environmental issues, restricted speaker capabilities and some of the artifacts of the necessary processing make them a compromise which is more easily overcome with discrete speakers , amplifiers and careful processing but at some cost and convenience loss.
