Why is this VoIP technology not a thing in most games?
Because nobody would use it.
Yeah, it's cool. It sounds like you're really in the game. You walk far away and your voice gets quiet... It echoes when you shout... Gunfire drowns you out... The radio has static and interference... It's cool and immersive.
Now imagine you're actually playing a game with another person, and you're trying to warn them about something up ahead... But they can't hear you because of all the gunfire, or there's radio interference, or whatever. Are you seriously going to put up with that?
Or are you just going to fire up TeamSpeak?
From a simple immersion/novelty standpoint, it's great. And as long as there's no actual tactical/gameplay reason to communicate with each-other, it's fine. I could see it being very popular in some kind of social setting, for example, where your voice is drowned out by the loud music in a virtual bar.
But as soon as there's an actual tactical/gameplay reason to communicate with each-other; that technology is going to go right out the window. As soon as people are trying to communicate enemy positions, or request backup, or shout insults at each-other, or whatever - they're going to get rid of the immersive stuff and go for the tools that allow them to communicate clearly.
There's already tons of options out there... Ventrilo, TeamSpeak, Skype, Google Hangouts, Facetime, Mumble... Nobody is going to put up with getting killed because they couldn't hear somebody warn them about the trap up ahead. Nobody is going to say "damn, that's cool, I died because this awesome VoIP technology prevented me from hearing your warning."
And it isn't like you can prevent this with some kind of anti-cheat technology... There's absolutely nothing stopping me from running Skype or Hangouts or whatever on my phone. There's nothing stopping me from installing TeamSpeak on a crappy old laptop. There's nothing stopping me from picking up the phone and calling you.
The only possible place I could see this happening is in the context of a regulated, competitive match... Like with cash prizes and whatnot... With actual human beings monitoring the whole thing... But, even then, you'd need to have each player in their own soundproof room for it to make any sense.