Referring to death as falling asleep is a metaphor and poetic language. It would be similar to asking “Why do we say we saw a sunset when the sun doesn’t actually set but our position on the earth has actually rotated away from the sun?” Its just a traditional symbolic wording that expresses how things appear to us. Those who have died physically look like they are asleep and from this comes the ancient (and Scriptural) custom of referring to the dead as asleep.
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