Because the land never waves back.

Copy paste from Google. Short answer, salt is one of the most abundant minerals on the planet.

Ocean salt primarily comes from rocks on land and openings in the seafloor. Salt in the ocean comes from two sources: runoff from the land and openings in the seafloor. Rocks on land are the major source of salts dissolved in seawater. Rainwater that falls on land is slightly acidic, so it erodes rocks