Because in the real world, it's NOT simple to move water around at all. Moving water around has involved some of the most expensive undertakings this country has ever attempted, and has been responsible for massive environmental damage and the disruption of the livelihoods of countless people.
Moreover, the water has to come from somewhere. If you hadn't noticed, the entire western US has almost no extra water. Precipitation is simply not refilling the original sources of Western water supplies. Maybe you think it's cheap and easy to pipe it over the continental divide, after somehow wresting water rights from people in the East. If so, you're an ignoramus.
And desalinization is totally unrealistic to address anything but urban water use, which is a drop in the bucket.
I don't know why you're surprised by "weird nastiness" over water rights. Civilizations all over the world have been highly protective of their water rights for millennia, and many wars have been fought over water. Fresh water is probably the single most important resource on the planet, and nobody is going to give up their water without a fight, even if they're not using all of it at this exact moment. There is simply not going to be any Kumbaya solution to these issues.