If the copyright owner as opposed to the copyright author does not atribute the work to the author then they are plagiarising the work.
Ther are many definitions of plagiarism, here are some in this article. One is “Take and use another person's (thoughts, writings, inventions) as one's own”
Here is an abrudged definition of Copyright, “is a type of intellectual property that gives its owner the exclusive right to make copies of a creative work, usually for a limited time. The creative work may be in a literary, artistic, educational, or musical form” From Wikipedia
Ther are many differences between copyright and plagiarism but for the purpose of this discussion the main difference is authorship or original creation as opposed to ownership.
The definition above of plagiarism uses the word person, where as the definition of copyright uses the word owner. Because copyright is basically a commodity, it can be sold, meaning that intellectual property can be owned by other than the person than created it. It could be owned by non person such as a corporation, association or government.
Under contract law it is possible to assign and or sell the copyright of intellectual property to a person other than the creator of the material. Similarly, under employment law an employee who creates intellectual property, usually, does not own it. It is owned by the employer.
Other than a few minor exceptions copyright law does not impose a requirement that the copyright owner, to state who the creator of the intellectual property is.
In those situations the owner of the copyright is plagiarizing the work of the creator. Without knowledge of who the author is, it would be assumed the owner is the author/creator.
Most modern politicians have speech writers. When was the last time you heard a politician say that someone else wrote the speech for me. Some of the most famous speeches in history may not have been written by those that gave the speech.
It's not very often that you hear CEOs and management levels personal, giving credit to the people that created the work for them.
Most websites do not say who created the work. Even in academia, other than technical papers, most academic websites do not say who created the material. They assume it's common knowledge
Often it is assumed that those that sings a songs, own them. Often this is not the case. The songwriter may be completely different to the song singer. When you hear a song played on the radio how often do they tell you who wrote it. Is this plagiarism by default.