Why do some Blacks folks say "You a lie" instead of "You're a liar"?

I'm Black, so no offense is meant by the question. I don't know if White folks have noticed this, but I'm sure that other Black Americans have. So anyway, why do some of us say "You a lie" instead of "You're a liar"?
Update: To ohno88 and Warrior for Christ....I am 100% Black. And I said SOME Black people...I didn't generalize.
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