There is this quote:
Expectations are the thief of joy.
Why is "thief" used in the sentence above and not "thieves"?
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Because the thing after the verb is a single entity. This often happens with verbs of identifying or equivalence:
My children are a delight.
Displaced people are a significant problem in the modern world.
Unwanted presents are a nuisance.
my responsibilities are a burden to me.
You could use plurals in the second half of any of these, but it is not usual to do so, and if you did, people would probably interpret as saying that they were several different kinds of thief, or delight, or problem, or nuisance, or burden.