I'll begin, but wait for other explanations... NEVER is negative, it means "not + ever" (opposite to "always"). You don't use "not" in a sentence with "never", because it's already in it: I never wash my hair on a Sunday I've never seen such a thing!
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Definition
ever (AT ANY TIME) [Show phonetics]
adverb
at any time:
Nothing ever happens here in the evenings.
Have you ever been to London?