Additionally, widespread use of web search engines has made URI stability less relevant for humans. Bookmarks are not the only solution to find a leaf page by topic again. A dedicated person might find that archiving websites may have preserved content at their old URIs.
Some of this is allowed to happen because the content is ultimately disposable, expires, or possesses limited relevance outside of a limited audience. Some company websites are little more than brochures. Documents and applications that are relevant within organizations can be communicated out of band. Ordinary people and ordinary companies don't want to be consciously running identifier authorities forever.