I just realized that there are questions and answers that are from "invisible users". I mean, I cannot find user information about them. There is no link to their page. See this question and the best answer. Maybe this is well-known to you but I didn't find the answer.
We allow unregistered users to answer questions - through unregistered, they can still set a user name.
It is also possible that the user asked/answered the question on another Stack Exchange site where it was not suitable and it got migrated (and the migration destination doesn't have this same user registered).
Additionally, some users wish their accounts to be removed. We can do this and still keep the posts by removing the account, but not the post.