For the same reason we can't drink seawater: there is too much salt in it. The recommended water intake for a human is ~2kg, the recommended salt intake is ~2g. That's 0.2%, seawater has 1.5%. After you'd have eaten your share of salt, there is some leftover. You can't have someone else eat it, because then you'd also need more water. You could try to export it, but that requires quite a bit of extra processing, and the global demand for salt isn't high enough to pay for the transport costs.