Well, Mirrors are viewed by Racing Types as something that unskilled cyclists use. Most mirrors have only a very minor weight addition. Racers crash fairly regularly and a mirror is just one more thing to break or get impaled on. Also most racers are young enough to be flexible enough to glance back and see stuff.
Mirrors make sense for recreational cyclists. I used to use a helmet mounted mirror and although those suckers are tiny, I could see all over the place by panning (moving my head slightly so the mirror would show me various angles). The mirror gave me great coverage.