That's because water is "the perfect solvent"--it dissolves soluble substances it comes in contact with, even glass--and the bodies of water that have been in existence the longest are the oceans. They're full of salt and a lot of other things.
Even rain, as it falls from the sky, dissolves whatever's in the air by the time it reaches the ground, but it's basically distilled by evaporation first and as precipitation, it's a newborn by comparison to the ocean. Its salt content is simply a matter of being a lesser degree.
9 months ago
Last edited at 6:57PM on 3/16/2013
Basically in simple terms, all water contains trace amounts of dissolved minerals and salts. And all rivers feed into the oceans. As water evaporates it leaves behind the minerals and salts. This process continued for billions of years. It believed if all the dissolved salt in the oceans could be extracted and spread evenly over the Earth's surface. It would be a layer around 500 meters thick.