For several reasons:
-In steamed milk you are not dissolving gases into it. You are creating an emulsion with the milk proteins and sugars. Water doesn’t have these, you need to dissolve the gases into water phase. Emulsions can be created with significant fraction of gas (lattes, etc). Without an emulsion, you are very limited. Think percentage vs parts per million.
-At atmospheric pressure and ambient temperature, you can only dissolve 20 ppm oxygen. Nitrogen is less soluble than oxygen. When coffee is cold and at 40 psig you might get 100 ppm nitrogen. At elevated temperature you have very little if any gas remaining dissolved at atmospheric pressure. Having used high pressure air and oxygens with sintered stones to aerate water, I doubt at brewing temps you are above 2 ppm.
-Ironically, you likely can have more nitrogen dissolved in your boiler. Cool water is saturated with the maximum dissolved nitrogen. It is then pulled into the boiler. Pressurized and heated. The boiler isn’t boiling the water (breaking out gases). It is still. Heated under pressure. For a given temperature, the pressurized water can sustain more dissolved gases. When it is used to fill a cup, it is now at atmospheric pressure and super saturated in those gases. If it’s filled gently, it can remaining supersaturated briefly.
-Steaming also strips dissolved gasses. This is common method industrial method to remove gas from water. You do not have the compounds to create emulsions. The physics of what happens when steam are completely different for water than milk.
I don’t doubt it tastes different.
But count me as skeptical that you can add any nitrogen to the water via steaming and agitating the surface of the water.