Because they use water for more than making steam. Much more water is used to cool the steam condensers and is often just dumped into the surrounding environment to cool off. Turkey Point in Florida has miles of canals that cool this water down.

If you don't believe me, then listen to the IAEA who created a water management program for just this reason:

Countries in water scarce regions, and considering the introduction of nuclear power, may show concern on the requirement for securing water resources to operate nuclear power plants and search for strategies for efficient water management. Experience has shown that nuclear power plants are susceptible to prolonged drought conditions, forcing them to shut down reactors or reduce the output to a minimal level.