I guess there aren't that many valuable minerals in seawater.
For example, Fritz Haber, a German Nobel Prize winner in chemistry, tried to extract gold from seawater after WWI to pay for the war reparations... long story short, the concentration of gold in seawater is too small.
Also, the phase transition for H2O from liquid to gas requires a lot of energy and space (evaporation surface). In other words, it takes ages to evaporate all the water. Also, the larger your pond is, the more expensive it is to scoop up the salt. And then just one rainy afternoon can set you back a lot.