Why do the two oceans not mix ? | Atlantic and Pacific | Gulf of Alaska

While we’ve given our planet’s oceans separate names, in reality there’s no border between them, and currents continually flow between them and mix their waters. The border between the Pacific and Atlantic oceans is like a line between 2 worlds. It looks as if the two oceans meet at an invisible wall which does not let them flow into each other and mix their waters. Why on Earth does it happen? We know for sure there is no invisible wall inside, and water is water