Because if theology is not constrained by Biblical revelation, you can pull whatever nonsense you like out of your head, and call the result Christianity.
Theology conducted without reference to the Bible is like science conducted without the annoyance of having to take physical reality into account. Fine if you are more concerned with doing your own thing than with discovering the truth.
To me, this is a non sequitur of sorts. First the analogy to science is (a) basically an attempted appeal to authority, assuming I (and others) trust Science to be the guide to accessing Truth, and (b) imprecise and perhaps inaccurate, as the scientific method of ascertaining truths cannot be separated from observation of physical reality (step one of the scientific method), while theology can be separated from biblical revelation, otherwise there would be no religion outside of and apart from Christianity.
I would agree that having an objective text allows the church to maintain neat borders between what is orthodox and what is heresy, and this has been the case for centuries. But would this argument make sense in the year 50 AD or 100 AD, or even 300 AD? Was the early church's theology based on biblical revelation? If not, were they not Christian?
While it is true that theology is conducted with reference to the bible (or other holy books for that matter), it is also true that the bible (and other holy books) were created by theologians, both amateur and professional) attempting to propagate certain dogmas within the church and attempting to root out detractors. It all seems very basely political, and I don't much trust the political process to get to the truth of a matter. It trust it more to obfuscate truth for selfish reasons.
I suppose the simple response, and the one I would expect, is that the Christian God guided the oral tradition, and chose those who would write the books that eventually became the canon. But that is assuming that God is the Christian God.
To me it is just as likely that Deist God had no oversight in the process. Assuming God had control over the situation and intervened in history would not rule out that God ordained the Inquisition or the Crusades or all of the Wars of Religion in Europe over the centuries.
It would also be just as likely to take a Jewish tack that Jesus was a fraud and that the Jewish God guided Judaism to avoid the heresy.
Finally, even if the Christian God guided any/all of these historical events, why would it be illogical to assume that He is now guiding Christianity, even you in particular, to a reinterpretation and adjustment of long-held views of Biblical hermeneutics or views of Christ in general. The argument, which at first presents as logical, would open up a whole host of other possible conclusions regarding God's will.
I once cherished the inerrant Scriptures, and as Charles Schultz once said, In all this world, there is nothing more upsetting than the clobbering of a
cherished belief. I'm not saying I am clobbering anything, and you can substitute in whatever verb you want.