Salt was NOT accessible everywhere. On the contrary, it was relatively rare.
Okay, so I'm writing a little unfocused as I'm awake since like 30 hours, writing my damn term paper in modern history.
Lets see if I can clarify what I mean.
In Civ you build your city where you want, because it can grow (more or less successfull) almost anywhere on the map to sizes, comparable to about many thousand or even million people.
In real life and history, greater communities needed strategies to stay alive, like consuming salt for making their food stay eatable a longer time, like you mentioned. Smaller bands of people where often nomadic or lived by hunting/fishing, so they consumed their salt through their food. The invention of agriculture brought problems with it, like bad harvests and almost no salt gain through eating the fruits.
So one can say: For establishing a bigger community (like a Civ city) having specificial natural ressources (salt) is a natural prerequisite. This is one of the many (although more bigger) reasons there weren't any cities in most inland regions of the antique world. Not until trade started to flourish or coastal empires (like Rome) started to expand into these regions.
There were wars for salt, yes. And there were and are big depots of salt which got these regions very rich in the middle ages (Alps, eastern germany, southern france, etc.) - but this was through trade and it was an purely economic feature.
So to implement salt properly into the game, one had to create a new ressource type, as we don't have economic ressources in the game at the moment.
One would even have to go so far, to implement mechanics like "your civ needs 1 salt for every 15 pop"... or similar, as it was a basic ressource all the time for growth.
So my initial statement "salt was accessible everywhere" should now be more clear.