Simple. You probably know why seawater is salty. That's right, because it contains salt (NaCl).
Why does seawater contain salt, but not rivers? This is where the confusion arises. Rivers and seawater both contain salt, just the concentrations differ.
The salt in the sea actually comes from the land. Before the Earth had water, it already had salt rocks. These salt rocks were once similar to the pebbles in front of your house.
About 4 billion years ago, as the Earth was cooling, raindrops appeared. Rocks were washed away by water, including salt rocks. Fortunately, salt dissolves easily in water, so the salts on land were eventually washed away.
The water containing salt then flows to a lower place, down to the lowest place. Sea. The sea water evaporated and returned to clouds again. So here the water evaporates, the salt remains in the sea.
Over millions of years, salt carried by rainwater accumulated in the sea. This is how we know seawater is salty.
This is a picture of a salt mine. These salt deposits are thought to have once been the bed of an ancient sea (uplifted by tectonic activity).
In fact, it's not just salt that's found in the sea. Urine, feces, garbage, and other waste also end up in the ocean.
Anyway, the conclusion is that sea water is salty because the salt carried by rainwater through river water accumulates there.