The total amount of water in the world does not decrease, but the availability of healthy and clean water does.  Once aquifers or glaciers are depleted, they are not easily (or at all) restored. Also, much wastewater is released out to sea, so does not re-enter our freshwater supply. 

Also, the more polluted the water source, the more difficult it is to treat the water, thus the more expensive. When water is polluted, sometimes treatment is not possible at all-- at least, not without the use of highly expensive systems. 

Finally, your postulate about governments charging more than private industry is not accurate. Case studies around the world show that private industry (because they are for profit) have higher water costs than public. In the US, private providers charged 59% more than public providers (see: http://www.foodandwaterwatch.org/sites/default/files/report_state_of_public_water.pdf). So, please check your facts!