Perhaps because in their current states headsets do very little to enhance the gameplay. As others have said, when you encounter someone with a headset your in most cases encountering someone who wants to spew hate or arrogance, A control freak who is obsessed with dictating other peoples play and takes a game entirely too seriously or people who confuse being able to talk to people online as a form of social interaction and friendship ending in VC about anything other than the game or the tactics involved. and the other seemingly only 1% who make up the people who use VC to enhance the game exp who inevitably realize that voice chat is laggy and delayed to the point that what is being said on chat channels is so far behind what is actually happening on screen or is so distorted into incomprehensibility (YAAAA 8) that it is nothing more than useless background noise.
Honestly, I dont get the desire to use headsets and voice chat at all. It really seems like a fad that had its time and is slowly passing away. Sort of like last generations motion control.
In the days since the original Xbox where headset use (at least for consoles) took off, we have improved VOIP technology greatly to the point it can basically replace a telephone, but yet for some reason Voice chat which uses VOIP still sounds static-y, crackly, distorted and delayed. Why is that? Because the people who develop the technology for the game realize that players are not using it for its intended purpose, so what is the point of investing effort into refining it?
Besides, even on a mulitplayer level, Games need to be solid enough to be playable without such communication first, and have VC as what it is supposed to be, an enhancement. To fail there is to leave a game broken and incomplete knowing large groups of people do not like VC and find it very counter immersive to a game play exp. Good examples of multiplayer that get this right are the Left 4 Dead series where everyone as an individual basically knows what they are supposed to be doing and need no such direction, or Demons souls where there is multiplayer and basically no VC at all, yet communication still exists.