There's this thread that answers your question fairly well. The gist is, people needed a LOT of it (100 pounds per year per person) and for most cultures boiling sea water didn't produce enough salt to pay for the fuel consumed by the fire. https://www.reddit.com/r/AskHistorians/comments/14v9f4/why_was_salt_so_expensive_in_ancient_times_close/

Side topic. When did salt become a tool for melting snow?

Boiling seawater is not a very efficient way to obtain salt, and historically it's not a common method of salt production.

Also, having read Salt (which is probably not a good historical source, but it is a reasonably OK culinary/cultural source), it's also clear to me that, for most of history, we weren't so much looking for just any old source of NaCl, but salt that tastes good (i.e. lacking impurities) and is easy to use for cooking. Certain methods of salt extraction are better than others, and certain places are better for doing this.

Not to mention, of course, that there are plenty of ancient cities that are inland and not near a good source of seawater. Rome, just to name an example I know a good bit about, is several miles inland. Nowadays "several miles" doesn't feel like a lot because we have cars, but it certainly would have been far enough from the sea that the average Roman couldn't just walk down to the Mediterranean and get buckets of seawater to boil down. It would be a commodity most people would have had to buy in a market.