what a person receives at Baptism, Mary received at her conception. From the first moment of her conception, she was free from sin – immaculate.
Well, there's no reason why we should. Because of the Grace of Christ which comes to us through Baptism, we are capable of never sinning again. The problem is, while Baptism takes away original sin, the effects of original sin continue to weigh us down such that we continually stumble in the face of temptations.
You are confusing personal sins with original sin. Original Sin is the "disease" that is communicated to us all through our original parents which banishes us from God's Grace and leaves us weak and helpless before the temptations of the Devil. The Life and Sacrifice of the Incarnation of God's eternal Son Jesus Christ redeems this state of being.
The Immaculate Conception states that Mary, through a singular Grace of Christ, was preserved from original sin at the first moment of her existence. Like Adam and Eve, she could have abused her freedom and sinned during her life, but she never did. She remained firm in Grace as it blossomed in her during her life - centered on her Son and Creator, Christ Jesus - on earth.
That is what this doctrine teaches. Christ's Church led by the Holy Spirit through the Centuries developed, and continues to develop, an ever-deepening understanding of the totality of Christ's Incarnation and redemptive Sacrifice, of which the Immaculate Conception of His mother is a part.