Because the Lord has established a Church to help determine what is and is not doctrine (Ephesians 4:11-14). Since 1835 (D&C 107) we've known that the FP and Qo12 are equal in authority and therefore, both bodies must agree on doctrine.
And where did Ephesians come from????? Yeah, you got it, from the same flawed source the rest of it did. Personal revelation.
Do you really not see how endlessly circular - and pointless - this all is? No personal revelation is reliable. You've already affirmed yourself that the personal revelations recorded in the Bible were not reliable. We all know that the personal revelations of LDS prophets and apostles are not reliable. Again, this helps you, how?
Which is why, for example, Joseph Smith had 11 witnesses for the Book of Mormon. For the crowning epiphanies, you have only the HG to bear testimony. It really is the great test, and everything else falls into place afterwards or it does not.
11 witnesses who signed a pre-written statement. Yeah, that is so convincing. To top that off, human memory is one of the most unreliable sources of information around. And the "great test" is to rely on the HG... which is the same source of all those other flawed revelations.
I wrote a lengthy post for FAIR some time ago about the unreliability of revelation, and how it dooms the entire foundation of the LDS church. I'll try to find it.
Chris:
I remember an article I published (when I was editing The Evangel) by a sharp guy (I'd have to look up his information) who argued that the LDS Church has only two ultimate, irrevocable doctrinal beliefs:
(1) That God exists
and
(2) That revelation continues.
Nothing else is, or, indeed, can be, sacrosanct.
This is basically Ben McGuire's position, if I've understood our past conversations correctly.