Why most of the microphones are placed on the left of headsets / headphones?

The cord on headphones is usually on the left, so it makes sense for the mic to be on the left too so that the mic's cable doesn't have to cross all the way over to the right side, thus reducing the wiring/complexity.

1. Should headset mic be on left or right?
2. Do microphones have left and right?

Should headset mic be on left or right?

Positioning a headset microphone

Place the microphone element to the side of your mouth to avoid noise from breathing.

Do microphones have left and right?

The microphone capsules are arranged in close proximity at an angle from 90 to 135 degree. In general, the microphones are panned hard left and right, with one microphone signal going to the left recording track and the other to the right recording track.