During earlier days, before ships had rudders on their centerlines, boats were controlled using a steering oar. As it is very common that most of the people are right handed in the world, most of the sailors were also right handed, so the steering oar used to control the ship was placed over or through the right side near the stern. Thus most of the sailors used to call the right side as the “Steering Side”, which soon became “Starboard”. The word “Starboard” is formed by combining two old English words: stéor (meaning “steer”) and bord (meaning “the side of a boat”)
As the size of ships grew, so did the steering oar, making it much easier to make fast a ship to a dock on the side opposite the steering oar, i.e, the boats/ships used to dock with the left side of the ship facing the shore/dock.
The original name of the left side of the ship was not “port” but rather the old English “baecbord.” This was probably referencing the fact that on larger boats the helmsman would often have to hold the steering oar with both hands so that his back would be to the left side of the ship/boat. After “baecbord” came “ladderbord” meaning “laden” (meaning to load) and bord meaning “ship’s side,” this gave rise to the starboard rhyming word “larboard.” As the time passed, it became evident that “larboard” is very easily confused with “starboard” during communications. Hence it was replaced with the word “port” as this was the side that faced the port or the dock, allowing cargo to be loaded or discharged.
So, that is how the terms “Port” and “Starboard” came in existence.
Since “Port” and “Starboard” never change, they are unambiguous references that are independent of a mariner’s orientation, thus removing the chances of any ambiguity and hence sailors prefer to use these nautical terms instead of left and right to avoid confusion.