Artificial gills aren't used for diving, not because they can't extract oxygen out of water, but because they won't provide enough oxygen for someone to breathe (only arround 1% oxygen in water instead of 20% in air). So they stick with oxygen tanks.
But why don't they use them both at the same time? The gills will on give a little bit of oxygen, true, but over time it will allow the diver to stay underwater longer with the same oxygen tank. If you add a rebreather to that, so only arround 5% of your air goes away every breathe, it will be even longer.