Why didn't Matthew just say: “Thou art "petros"?

Why didn’t Matthew just say: “Thou art “petros” and upon this “petros” when translating the Aramaic used at the time of Christ since the terms were cognate at the time of his writing?

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Recommended Reading:
[LIST]
*]Peter and the Papacy
*]Peter’s Authority
*]Peter the Rock
*]Peter’s Successors
[/LIST]

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