They are empty places. The biomass per square area is a pittance compared to any other habitat. Water is life. Places with less water have less life. Not all places are equal.

A chicken farm has more biomass per square meter than just about anything else, but I wouldn’t advocate replacing the Mojave with a chicken farm. There is simply more to it than that.

Places with less water have less life.

This is only true if every piece of life requires the exact same amount of water. Which is false. Your argument easily falls down simply by noting that a river has less life than a rain forest.