In truth, you have a point here. However, since the point was rhythm, the words had to keep the attention of those listening and the chorus, sung by the crew, had to be repeatable. This is not necessarily clean or meaningful. As most of the chanteys we use are derived from the English tradition, the chanteys and choruses can be derived from shore songs, be simply space fillers with nonsense syllables or, in the later tradition, can help move the song forward. The entire point is that, like any other shipboard tool, it should be something that the crew could use.