Perhaps the Valar knew that Sauron would be defeated by men. We can be fairly certain that Eru knew what the outcome would be, so maybe he bid the Valar to wait, send the Istari for aid, and let the men and dwarves win their own war, as he knew they would.
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"'You," he said, "tell her all. What good came to you? Do you rejoice that Maleldil became a man? Tell her of your joys, and of what profit you had when you made Maleldil and death acquainted.'" -Perelandra, by C.S. Lewis
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