Most Helpful Guy
Because it gets bad. There are bacteria in it that constantly change the product. Also you can't get rid of 100% of all mold, one or two spores will be in it. After too much time it will grow to an unhealthy amount.
Now, that salt has an expiration date is just stupid. It lay millions of years around, and 3 years after being sold it goes bad? That's beyond stupidity.