Because Scripture is Scripture and Sacred Tradition is Sacred Tradition. Even though both of them are the Word of God, they are two entirely different means of transmitting that Word. The inspired Word of God in it’s written form was completed at the death of the last Apostle.
Here is how the Vatican II Document *Dei Verbum *explains it. Please read the whole thing carefully:
- Hence there exists a close connection and communication between sacred tradition and Sacred Scripture. For both of them, flowing from the same divine wellspring, in a certain way merge into a unity and tend toward the same end. For Sacred Scripture is the word of God inasmuch as it is consigned to writing under the inspiration of the divine Spirit, while sacred tradition takes the word of God entrusted by Christ the Lord and the Holy Spirit to the Apostles, and hands it on to their successors in its full purity, so that led by the light of the Spirit of truth, they may in proclaiming it preserve this word of God faithfully, explain it, and make it more widely known. Consequently it is not from Sacred Scripture alone that the Church draws her certainty about everything which has been revealed. Therefore both sacred tradition and Sacred Scripture are to be accepted and venerated with the same sense of loyalty and reverence.
Sacred tradition and Sacred Scripture form one sacred deposit of the word of God, committed to the Church. Holding fast to this deposit the entire holy people united with their shepherds remain always steadfast in the teaching of the Apostles, in the common life, in the breaking of the bread and in prayers (see Acts 2, 42, Greek text), so that holding to, practicing and professing the heritage of the faith, it becomes on the part of the bishops and faithful a single common effort.
But the task of authentically interpreting the word of God, whether written or handed on, has been entrusted exclusively to the living teaching office of the Church, whose authority is exercised in the name of Jesus Christ. This teaching office is not above the word of God, but serves it, teaching only what has been handed on, listening to it devoutly, guarding it scrupulously and explaining it faithfully in accord with a divine commission and with the help of the Holy Spirit, it draws from this one deposit of faith everything which it presents for belief as divinely revealed.
It is clear, therefore, that sacred tradition, Sacred Scripture and the teaching authority of the Church, in accord with God’s most wise design, are so linked and joined together that one cannot stand without the others, and that all together and each in its own way under the action of the one Holy Spirit contribute effectively to the salvation of souls.