Because it’s full of salt. Rain comes from evaporation which seperates out the salt, and natural springs filter the salt out through all that earth.
Sodium, Na, and chloride, Cl are abundant on earth and naturally bind together making sale. The presence of these two elements on earth largely corresponds with the [distribution of elements in the universe](https://en.wikipedia.org/wiki/Chemical_element#Origin_of_the_elements). Also, any salt that the rainwater and such picks up and flows into the ocean largely stays there. It’s kind of a one-way collection. Salt mines are sections of ocean that got trapped and dried out.
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