Because port/starboard refers to the left/right side of the ship, regardless how the person orientated. If you are facing the front of the ship, then yes, port = left. If you are facing the back of the ship, port = right. So it is very handy to have words that always refer to the same part of the ship.
(All the other terms, like front/bow and stern/rear, who cares? I dunno.)




In a car, sure, since 99% of the seats face the front. Same for an airplane. Left is left, right is right.
How would you do that with for instance a classic rowboat? Then your left is on the right side, no matter how you turn your head?