Say rain water is leaking into your house from two holes in the roof: one is 1 meter in diameter, the other is 1cm. Any effort you spend plugging the 1cm hole is a distraction from the 1m hole you really need to focus on.
You could make an argument that agriculture is different because we need agricultural products to live. But we don't need those specific products to live (alfalfa, almonds, etc), and they could be grown far more efficiently if water were priced by market rates.