Really? You must not have looked very hard. Even back when I went to high school (90s) it was possible to find complete book reports for just about any book online.
And some of the programming courses at my University used the same assignments every year. Finding last year's answers was usually trivial.
Then there's the fact that nearly all programming assignments allowed you to work in pairs. This usually meant that one person did the work for one class, and the other for another class. This means that there's one subject that each of the two never actually did assignments for, and thus wasn't tested in.
If two students come up with the same Haiku, they can share credit,..
And what if they both claim to have come up with it independently? You have to give them both full credit, otherwise you're punishing them for cheating, which you want to explicitly allow so you can't punish them for it.