A few observations to add to the pot:

Whenever men/boys are kept away from restraining influences for lengthy periods of time in largish groups they tend to promote material that is of a less restrained nature. This will include all manner of folklore types, stories, songs, jokes, rhymes etc and cover all sorts of subjects, sex, scatology, racism, blasphemy and other forms of prejudice. Lying embedded in this it can also include the most sentimental drivel, and some of the most beautiful sentiments. Such happens in the forces, schoolyard, team dressing room, the men's bar etc. Why would being at sea be any different?

In answer to my own question, one, where the majority of the group were heavily religious.

The question was put why some chanties were pretty standard texts. In many cases the working conditions of the crews wouldn't allow any thinking time for making up new verses, particularly ad hoc, and particularly for the short haul type when the sole purpose was to get the job done as quickly as poss under difficult conditions. The slower, more mundane tasks, maybe.

Isn't Amsterdam one of the earlier chanties? The bawdy verses must have come first as they are part of the shore songs it originated from.

The forecastle songs must have included much that was bawdy, scatological etc and in the main (excuse pun)they could have got that out of their systems in that context.

The worse forms of scatology and filth I have ever seen come from printed material from the 17th and 18th centuries and the people who sang and were entertained by this must have been pretty literate.