because the further the microphone is from your mouth, the more sensitive it has to be to pick up your voice properly. The more sensitive it is, the more noise it picks up along the way. Even if the microphone is very directional (which I think is hard to do properly in small footprint), it will pick up everything that is in front of you because it has to be directed at your mouth.
I’m by no means a fan of steelseries or razer or any of the other “gamer” crap, but can’t deny this one is a smart microphone design (as long as it doesn’t break):
You just push it in when not using it, so you don’t look like tech support, but when you do use it, you can count on a good voice quality and not worry about external noise too much. After all, these are “noise canceling” headphones so that implies they will be used in noisy environments.