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.
The most Blessed Virgin Mary was, from the first moment of her conception, by a singular grace and privilege of almighty God and by virtue of the merits of Jesus Christ, Saviour of the human race, preserved immune from all stain of original sin.
--Pius IX, Ineffabilis Deus (1854)
In theory, you are correct - but once we reach the age of reason, we need the Sacraments to help us remain free from sin. Without the Sacraments - particularly Confession and Communion, its doubtful at best, if we could remain free from sin for very long at all. As TotusTuus points out, even though we get baptised, we still suffer the effects of Original Sin, other wise know as concupiscience or "the tendancy toward sin".
Our Blessed Mother, being born without Original Sin, was not subject to concupiscience. Our Blessed Mother, lacking the essential ingredient for sinning therefore lacked its effects or its tendecies - rather, Her tendancy was always toward God and growing in His grace.
That is perhaps the most difficult thing for us to imagine because we struggle to remain in the state of grace, but Our Lady constantly grew in the state of grace.