Are you saying they aren’t?
A copyright is secured automatically when the work is created, as long as the work contains a sufficient degree of originality
Source: Copyright basics | USPTO
@Teafh You can have a look into “collective work”, which a wiki probably is. You usually have an history and all the revisions, so it all works fairly well. But, it’s an interesting question in case of deletions. Basically: Should the initial author have the right to destroy and delete the work of all other people?
If you take the example of the US Copyright Act of 1976, its Article 201 (c) states:
Copyright in each separate contribution to a collective work is distinct from copyright in the collective work as a whole, and vests initially in the author of the contribution.
To answer your question, I guess you would need to keep somehow all the portions which have been contributed by other people in order to allow them to republish it, for example. When deleted, the wiki post should be archived somewhere, and only what has actually been contributed by the initial author should be deleted. I don’t think that’s possible or been taken care of at al by the software, at the moment.
Or you would have to go with some rules on your end. Like for example “a wiki post can’t be deleted anymore” (the initial author could still edit out all of his work) or “all contributors have to agree”.