Why Is The Sea Salty, But Not Rivers And Lakes?
It all comes down to a thing called the water- or hydrological cycle.
Rain, which is fresh water, falls from clouds onto the land and finds its way into lakes and rivers, and also through the ground, back to the sea, picking up salts and minerals as it goes.
Once it reaches the sea, the water can be evaporated again to form new clouds containing fresh water, and the salt is left behind; so, over millions of years, the oceans have slowly been accumulating salt washed off the land by fresh water.
So is the sea becoming more salty now?
Probably not, because if the level of salt rises any further the extra is removed by various processes, including chemical reactions, so the sea is now about as salty as it is going to get.
That's not to say you can't get saltier seas – like the Dead Sea: these are just bodies of water cut off from the main ocean and in which more water is evaporating than being returned by rivers, so the water becomes more concentrated, or saltier...