Why does saltwater and freshwater not mix?

Osmoregulation is how Saltwater Fish breathe in salt water. They can actually live in fresh water, but would be incredibly stressed. The better question would be why cant fresh water fish live in salt. Marine fish evolved to retain and release salts in their body to keep their body salinity level perfect no matter what the water conditions are. Whereas a Freshwater Fish, can not adapt to salt because it takes on the salt threw the gills and can not release its own salinity level to balance. So their bodies literally hemorrhage from the high salt level