[1] https://en.wikipedia.org/wiki/California_State_Water_Project
Then, desalination requires energy, and Iran already faces blackouts here and there, there just isn't much spare capacity.
That is enough for drinking and probably enough for cooking which should be the priority in a situation like this.
Or the undground Great Man-Made River Project of Libya moving 6.5 million cubic meters over 2,820 km.
Main issue ther though is the first is from already present freshwater sources and the latter from underground aquifers. With both having been done over multilpe decades to reach that capacity. Finding the water to move would be the main challenge, een though the Caspian is less saline than ocean water - there are probably water usage agreemets with the neighbourign countries preventing a massive undertaking of such size.
This situation was avoidable but it required investment years ago. Kind of too late now.
Nearby Israel has desalination plants that seem to be working out well.