I reckon Morgoth didn't send out constant plagues because he had already spent too much of his power on orcses and dragons. He would have if he could as he was a nihilist who wanted everything and everyone dead.
Sauron might have had a similar lack of power but also he was content with creatures living as long as they were his slaves to do his bidding.
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As Beren looked into her eyes within the shadows of her hair,
The trembling starlight of the skies he saw there mirrored shimmering.
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