Because they directly lead to water shortages and droughts, as Kathryn Muratore at Mises.org points out.
“But wouldn’t companies charge super-high prices for water?” is the common retort.
My response: Where do they do that with any other natural resources critical to a modern economy? Isn’t it rather government products, resources and services that are higher priced and of lower quality?
Ludwig Von Mises wrote in his book Socialism that socialist governments and programs eventually fail because they are unable to determine price or have a price structure. Among other things, a customer is necessary to figure out the costs and therefore the price of something. Without it, you cannot determine where to allocate resources or how to use them, as you do not know where they are best used. This is why socialist economies are doomed to fail. Some take a long time. Others take longer. But they are not sustainable.
They also lead to shortages, which is why for years during the Cold War the United States exported wheat to the Soviet Union, an empire that included Ukraine, long considered the “Bread-basket of Europe.”
Government legislation like the Homestead Act of 1866 helped to cause the situation we have to today by encouraging people to build homes and farm in areas where there was not enough of a water supply to sustain crops. Rather than let the free market determine where farms would be built and crops grown, the government under Roosevelt offered irrigation subsidies to farmers. Subsidies continue to this day and remain an entrenched aspect of the federal budget.
As for how private ownership of water would work, Muratore explains:
Privatization does not mean selling dams and reservoirs and municipal water companies to private investors who are then allowed to operate as protected monopolies. Privatization means that you own the rights of the water on your land, and individuals can own real estate in water (lakes, rivers, oceans). A transition to such a state of affairs is likely to be painful, as 100+ years of damage by socialization of water must be undone. But, I am certain that the process will be relatively quick compared to the long, drawn-out suffering that is in store if water socialism is allowed to continue.