Why? There's nothing in Scripture that says a sinless nature is a prerequisite for giving birth to Jesus Christ.
Instead, that is part of the miracle -- that God became man and was born of a woman; a woman graced by God, but nonetheless a fallen human being in need of a savior, just like the rest of us.
Our interpretation of sripture says otherwise.
You are absolutely correct. The word translated "full of Grace" in Catholic Bibles and "highly favored" in many other Bibles is Charitou. It is found one other place in Scripture: Eph 1:6 To the praise of the glory of his grace, wherein he hath made us accepted in the beloved.
Where it is found is not in the glory of His grace; rather in our being made accepted.
Mary was made accepted by God. She was just like us, but as part of the miracle of the Incarnation, God made her accepted to conceive, carry, and birth Jesus.