The magisterium has the responsibility to be the teaching arm of the church..isn't it strange that in 2000 years they NEVER wrote a complete official commentary of the scriptures.. This allows every commentator to be his own personal interpreter of the bible.. exactly what catholics accuse protestants of being
That is opposite to how the Church works. At the start, the Apostles verbally handed on the teachings they had received directly from Jesus and from the Paraclete, mostly through councils but perhaps sometimes individually. The Gospels, Acts, and Apocalypse formed the basis of written teaching but most everything else came as a response to problems that required fixing (or heresies that required suppression). The Church can never undefine a teaching she has infallibly defined. Therefore, the Church only defines doctrine when it is absolutely necessary.