The illusion of soundstage.

Or, in my case and situation, I'm running Walsh drivers which are omnis.  I've a 'soundstage' that 'extends' not only to the sides but also in front and behind the drivers.  Placement within the given space and the surrounding objects (or lack of same) will certainly effect 'the effect'.  I've also noted that from selection to selection, artist to artist, album to album there's distinct differences in what is perceived where.
Adding a second pair, either in 2 channel or 4 channel arrangements, and playing with delay or no, you can be sitting 'on stage' or row 5....
But 'immersion' isn't to everyone's taste.  To each...

erik's got it sussed, and geoff's got the clues....HRTF is why headphones work so well, and the 'ambient' (whether real or 'created') is what we experience with any given pair of speakers to a greater or lesser degree.  And then we devolve into where and how our speakers are placed in a given space, what the space consists of  (what's in it, what's not, how that's arranged, hard/soft surfaces/materials, ceiling height, width/depth, toe-in/toe-out (sounds like Arthur Murray to me, two, three, four...), what you've drunk or not, etc., ad infin...

You either go bonkers with creating the 'perfect' space, which will Still be a relative rationalization unless you're an audio engineer with a talent for spatial calcs of a mind-breaking sort and a better than modest budget OR you can 'ignore the room' (per Linkwitz) which is where I'm at (it's a leased space that I can't go nutz in, and I'm cheap) OR you can do the best you can with whatcha' got with the furniture that's there and the SAF.

In the nutshell most of us occupy, that's the devil's deal.  My omni's complicate the issue, given what they do in a given space.  So I play with active eq, a tad of delay, and strike a deal with the space which is Very 'Live' (everything is 'hard' and unforgiving...).

It works well enough.  Been in worse, been in better....

tostados, agreed, the engineer 'sets the stage'...virtually, but that's the modern recording studio for ya....not to mention the other 'enhancments' that can be dialed in.

I suspect that given current tech might allow a certain level of 'ambiance' in a mono recording that comes through with a high quality system, but I've never had the pleasure to hear such.  On the other hand, I'm not disposed to go looking, either...*G*