Humans rarely use new words when old words will do. When we were looking for a word to use for "thing that keeps us alive in space", "ship" was the obvious one

Humans rarely use new words when old words will do. When we were looking for a word to use for "thing that keeps us alive in space", "ship" was the obvious one since we already had a word for "thing that keeps us alive on the ocean". When we are looking for a word for "soldiers that accompany ships in space", "marines" is almost certainly the word that is going to be used.