The bubbles still dissolve some gasses in the water as it is still air passing through the water. If you watch a large bubble very carefully, or see it filmed and then shown in slow motion, you can see that an air bubble doesn't actually move independently through water without breaking the surface tension of the water. The air actually pushes up through the water, breaking the surface tension and displacing the water directly above it. In slow motion you can actually see the water being displaced and cascading around the air that forms the bubble. For this reason, bubbling is a very effective method for dissolving oxygen in water.
But cascading, surface bubbling, and any other method that breaks the surface tension of water will also effectively aerate water. The problem with something like top drip or DWC is that there is little to no fresh oxygen getting to the reservoir where good oxygenation is needed to prevent anaerobic microbe growth. Having a pump constantly pushing fresh air into the reservoir eliminates that problem