Why there are not more desalination plants for fresh water as oppose to sucking the groundwater dry?

Recently 60 Minutes reported that thirsty economies are literally sucking the planet dry; depleting the groundwater. To do so is supposedly heading the planet for terrible times in the future as these underground aquifers either cannot catch up with the rainfall or takes decades or centuries if very deep. We have more than 50% of the planet as water in the oceans, science is not smart enough to find a cost effective way to use the water right in front of their face as oppose to water with limits on the bowels of the Earth? Even if the initial cost was more than people wanted to pay, decades down the road would it not prove to be a good investment, if we truly want to leave the world better off for future generations? Today with all this technology the science gods, still cannot figure a way to cheaply get the job done?