From the point of view of Australia having water locked into glacier instead of raining down on our farmland is a crisis.
So if we all start geo-engineering rainfall on a global level what happens when one country wants water that other countries also want? What stops us geo-engineering our deserts to steal your rain? Who sets a quota describing how much rain we're allowed to have, and how will that be enforced?
There are some big technical problems with this plan, but there are also massive social and political problems to be overcome also.