Stories often have prophecies using convoluted ambiguous phrases, unclear wording, or oddly poetic form. Things like saying "no man may defeat me" only to have the person defeated by a female dwarf, or "The greatest gift of humanity shall be discovered by the one who learns the see the dark lord's true form" and the great power turns out to be falling in love with the dark lord, instead of some powerful magic like every expects.
They don't have to be plays on words, prophecies could simply be so poetic or figurative it's hard to understand their meaning, or leave out important context, or say something is important without saying if it's good or bad etc, the point is there is something about them unclear enough that it's not easy to act on them.
What is a good way to justify these prophecies existing as they do, as opposed a more clear and exact prophecy that is easily understood and more helpful?
I'm looking for more than "A wizard did it." It's not enough that the magic of the world makes it happen this way, why would the magic work in this way? Assume prophecies are not being written by a god or similar all knowing being with the intent of manipulating actions of regular humans. Finally, assume a prophecy is not an absolute prediction of the guaranteed future, how one acts on a prophecy may affect it, if a prophecy says a great army will come from the north and destroy everything you can fortify your north border in hopes of driving the army back and surviving for instance.
Why would humans create prophecies that were so hard to understand, instead of simple prophecies such as "to stop horrible event X just do simple action Y" sort of prophecies?