Generally speaking, plagiarism (as in, passing off other's words and ideas as your own) is not illegal except in cases where it intersects with copyright law. In the case of ghost writers, they transfer copyright to the publisher, so legally speaking there's no issue.

In academic writing, there's a standard that the authors are the sole authors of the text that is upheld by academic institutions, academic communities, and the academic publishing industry (but not the legal system itself). This same standard doesn't exist in the same way in non-academic publishing (or a lot of other media) so not crediting ghost writers is considered acceptable.

General books are not the same as academic papers.

(I am aware there are academic books)