Presumably because they do not want huge segments of the human population to die because they are unable to afford access to the literally most basic necessity for life on this planet.
Funny thing. There's a research topic called virtual water that focuses on the trade of water through food or other "products". Look it up! It's really interesting! It talks about the trade of water most commonly through food products! (How transporting an apple from California to Pennsylvania means you're exporting and importing water!)
It's a new topic that's only really taken shape and impact in the last two decades!