Because stereo is stereo? Left and right, that's it. There's absolutely nothing in there for vss to work with. How would it know that a sound is behind you when it's physically just on the left or right? It's like asking someone who has never seen a Monet painting to recreate one using only a black and white picture of it, the best he can do is guess the colors. The result may be pleasing to the eye but far away from reality. The way vss works is by tricking our brain using HRTF algorithms that (try to) simulate the way our ears work by changing the way different sounds behave. It can be accomplished directly within a game's sound engine (and there are very few games doing that, origins and wildlands are not part of the list) or with a third party vss solution (like dolby atmos, windows sonic, gsx, sbx, bxae etc...) which would take a 7.1 signal, with 8 separate channels, create a virtual room with 8 virtual speakers, and then mix everything into a modified stereo output. With just plain stereo the room is still there, that's the sense of "holographic soundfield" you're getting, but you only have 2 sources. It's no different from just sitting in front of your computer with a pair of speakers.