Transporting any significant quantity of sea water would have been a serious challenge to the ancient world, even just from the shore in to the middle of a port city. The easiest way to transport water is downhill, which, alas, doesn't work so well if you're starting from sea level.
You can get water from the seashore just by evaporating it. It's simple, effective, and was done. But it's a lot more convenient to swing a pick and get a chunk of salt the size of your hand, rather than secure somewhere to evaporate off water, load it up, wait for it to evaporate, and collect it. It's the same reason we're stuck on fossil fuels... it's not that they're the only method we have for obtaining energy, it's just that nothing else can compete with putting in 1 unit of energy and getting back 80 or 90.
"For a society without trains or trucks, moving bulk materials of any kind over long distances is extraordinarily expensive. Moving grain overland, for instance, would cause its cost to double after 100 miles."
You can get water from the seashore just by evaporating it. It's simple, effective, and was done. But it's a lot more convenient to swing a pick and get a chunk of salt the size of your hand, rather than secure somewhere to evaporate off water, load it up, wait for it to evaporate, and collect it. It's the same reason we're stuck on fossil fuels... it's not that they're the only method we have for obtaining energy, it's just that nothing else can compete with putting in 1 unit of energy and getting back 80 or 90.