Why not make a big solar still thus: go to Saudi Arabia, dig pit near the ocean, pile up the fill next to the pit, let solar evaporation carry freshwater up to

Why not make a big solar still thus: go to Saudi Arabia, dig pit near the ocean, pile up the fill next to the pit, let solar evaporation carry freshwater up to the top of the pile, store it on top so you have pressure? There must be a reason why not make lots of these on the coast. If someone knew how to run the numbers I would be very curious.