Doesn't it evaporate on its own over time?
You can look at salt pans on Google Maps. Here is one I have been to:
https://goo.gl/maps/uN968Stn6kPHnH6E7
Obviously today this is done with gigantic machines, and the cost/kg is incredibly low to the point that locals do not pay for salt, but back before the advent of Caterpillar, I imagine harvesting salt this way was very labor intensive.
There is a documentary on Amazon called My Name is Salt about the salt harvest in India. It is quite good.
Other than needing some decent sunlight and not super-high humidity - what am I getting wrong?
And that heavy rain (monsoon, etc) will wipe out all your work.
Heavy rain would indeed wipe out all your work, which is why you would do it in the drier seasons.
It certainly was done. Looking back at history, the places that could efficiently do so were major salt exporters; which rendered it uneconomical to do elsewhere.