Here is the story of why sailors started using those terms.
Why they still use them can probably be chaulked up to tradition.
The old Viking ships were steered by rudders on the right side, which the Vikings called a styrbord, Scandivanian for
The term was adopted into Old English as steorboard, which later became starboard.
With the steering on the right side, the Vikings docked on the left side and so called it ladebord, “loading side,” which the British turned into larboard.
Unfortunately, starboard and larboard sound enough alike that British ships had some fatal confusions.
The Admiralty ordered that the left, docking side be henceforth known as the port side.