In addition to what's already been said:
1) If you make the ring big enough to be visible, it will also be big enough to seriously snag on clothing in a CCW quick-draw situation. (This is admittedly a problem with many conventional notch-style adjustable revolver sights too, but it would be worse with a ghost ring becuase it would stick up farther.)
2) Ghost rings require too much concentration and time to use in combat shooting. Notch-style or 3-dot sights can be lined up faster, even if they're just being roughly lined up in a quasi-point-shooting SD scenario.
3) Most handgun shooting is done at ranges so close that the added precision would be of little benefit.
All that said, I could see them being useful for handgun hunting with a long-barrelled revolver because the guns have a longer sight radius, ranges are generally longer than in combat shooting, and the ability to aim quickly is less critical. However, since American rifle hunters traditionally prefer optical sights to complex open sights (just look at the racks and racks of scopes at your local sporting goods store), the gunmakers would probably hesitate to offer a ghost ring on a production handgun when they can just slap on a scope rail instead.