This is primarily because rivers continuously receive fresh water from rainfall and melted snow, diluting the salt content. Most lakes are also not salty for similar reasons. They receive freshwater from rivers and precipitation, which dilutes any salts. However, there are exceptions.

Oceans are salty because they act as a "final destination" for minerals, whereas most lakes have an outlet that allows salts to be flushed away. Over millions of years, rain—which is slightly acidic—has weathered rocks on land, carrying dissolved minerals and salts into rivers. These rivers eventually dump their tiny salt loads into the ocean. While the salt concentration in a river is too low for us to taste, the ocean has no way to "empty" itself; the only way water leaves the ocean is through evaporation, which leaves the salts and minerals behind to build up over eons. Most lakes remain fresh because they have a constant flow of water coming in and an outlet (like a river) leading out, preventing a buildup. However, "terminal" or "landlocked" lakes like the Great Salt Lake or the Dead Sea lack an outlet and therefore become even saltier than the ocean as water evaporates and minerals accumulate, proving that the lack of an exit is the key to salinity.