The Bible is the Word of God, but that doesn’t change the fact that it is still a book. It does not “speak” to us in a real way in order to tell us what it means. It is true that in some cases we can get a better idea of what some passage means by understanding it in terms of what some other part of it means, but this is both limited and error-prone. It is limited in the sense that there aren’t always other passages to explain the ones with which we have difficulty.
It is error-prone in at least three ways. Let’s say that A is the passage we hope to understand and B is some other passage in terms of which we seek to understand A. The first way in which using Scripture to interpret Scripture is error-prone is that there is no guarantee that B is the correct passage in terms of which we ought to understand A. Consequently if we err in selecting B as our interpretive guide for A, we will arrive at an erroneous interpretation of A.
The second way it is error-prone is infinite regress. If B is necessary to the understanding of A, there is no necessary reason why some other passage C would not be needed in order to understand B, nor a D for C, and so forth. There is no way of knowing when or whether such a regress might end, and consequently the potential for error is injected into our attempt to understand A.
A third way that it is error-prone is that we have no means of knowing whether B really is needed for understanding A. It might be the case that I am simply not sufficiently gifted to properly understand A, and by interpreting it by means of B I might get A wrong.
It is hopefully obvious that the entire endeavor of interpreting Scripture by Scripture is not just error-prone; it is also substantially dependent upon what we put into it. Baptists interpret passages Baptistically; Presbyterians do it with a Presbyterian flair; Methodists have their own method. All this amounts to one reason why we need an authoritative interpreter for Scripture, and why we need to read Scripture according to the Sacred Tradition of the Church rather than according to our own lights.
Bryan Cross has some very useful things to say about this in his comment here:
Books do not have unlimited potency with respect to interpretive self-clarification. And because books don’t have that, they cannot function as interpretive adjudicator when there are competing interpretations facing the Church: each side can appeal to the book to support its own position, and without a magisterium, the disagreement can be a perpetual deadlock or impasse. But a living magisterium can not only adjudicate an interpretive dispute, it can also provide clarification regarding previous statements or judgments it has made. That is why having a living magisterium does not leave us in the same epistemic quandary that we would be in if we had only a book and no interpretive authority.
One analogy might be found in the position of the U.S. Constitution. It’s a mere document. If we did not have a Supreme Court to adjudicate disputes related to constitutional matters, we would have anarchy: groups and individuals within the government would be forced to decide for themselves what the Constitution means. We need a final authority for resolving such conflicts, and that is why we have and need a supreme court. Similarly then, we need a Magisterium to adjudicate concerning the content and meaning of divine revelation. It’s not that the Magisterium is above revelation; it’s that the Magisterium is tasked with safeguarding it.