For starters, lakes and rivers do contain salt, just not as much as the oceans. A large portion of those salts and minerals washes downstream into other rivers, or through the outlet stream or river of a lake, and eventually winds up in the oceans. So, the answer to why rivers and lakes are not as salty as the oceans is that salts and minerals that enter have an avenue for escape, which is a path to the oceans. Oceans don’t have an outlet though. The primary way that water leaves the oceans is through evaporation, and that process leaves salts and minerals behind. No outlet means a buildup of those things, and a salty ocean. So, it appears that the oceans began as slightly salty, and the “fresh” water from rivers just continues to make them saltier.