You're talking simulatneously about games that "have 5.1" and "games that are immersive with the 5.1 functionality providing actual gameplay feedback".
They're not the same and you know it.
Most of those games "in 5.1" are just munged stereo extended out for a couple more channels. And the cost of developing these two types of "5.1 compatible game" are very, very different.
No, I'm not.
I am talking about 5.1 surround period.
If I stand in front of a fire, I hear that fire in front of me. As I turn around 360° I will hear that fire travel around the room accordingly.
If I am racing down the street at 150mph in a rear engine car I hear that engine predominantly in the rear. I hear the car I am passing on my left go from the front left channel to the rear left channel.
As I am peeking around the corner, and I hear footsteps behind me, I know I may be in trouble, and I know exactly where to look first.
When I am online playing _______, I can hear where my enemy's gunfire is coming from. I am not just surprised by a loud bang, but I know exactly which way to look to return fire in case I lived.
As I am snowboarding down a mountain, and a snowman calls out to me (it's an Amped thing

), I can pick out exactly where on the mountain it is coming from by the direction I hear his voice.
I know what discreet surround sound is.
I am not talking about 4-5 channel stereo. I do know the difference.
Most games had real discreet digital surround sound on Xbox last generation. they have had it on PC for years even before that. They have it on the 360 and PS3 this generation.
That doesn't mean there weren't games that didn't have it, but they were the minority, and will only continue to be such.
I don't know what kind of development you do do, but it does not seem to be modern game development.
I don't develop games, but I sure as heck can tell when something is using real surround sound. As much of a common place thing as it is, it just doesn't seem to be as hard to implement as you make it out to be.