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Here in the Virgin Islands, much of our personal water consumption comes via rainfall we capture from our roofs into our cisterns (big underground swimming pools). Even with that I still need to truck in water a few times a year when the rain hasn’t been sufficient.

My roof surface area to consumption ratio is such that this system works pretty well for me (big roof, small human). However with a farm the roof surface area to consumption ratio is flipped - big fields needing water and little to no roof (maybe a tool shed or garage). And you can’t increase roof surface area since that would block sun from the plants. Industrial uses also have a poor ratio. Dense usage of water in a limited size building.

Yes indeed, much easier. The problem is that the places where you need desalination tend to be the places without much rain.

There's actually water rights/legal issues to this. For example until recently it was illegal for me to do this in Colorado; the law has changed to allow ~114 gallons/year.