Because I read the Bible wherein I learn that it is the word of God. Further, it instructs me (and all Christians) to preach the Gospel to all men.
If the Bible does not interpret itself, then whose interpretation is authoritative?
I just said the Bible does interpret itself. Men compare Scripture with Scripture and thus learn God's meaning. God gave us ears to hear and quickened minds to understand in order to rightly discern His word. Some will get it right, by the grace of God, and some won't. Regardless, the infallible measure of Scripture is Scripture because God has told us it is through His word that the Holy Spirit speaks to us and guides us in all righteousness.
Unless you don't believe the word of God.
"Even the Spirit of truth; whom the world cannot receive, because it seeth him not, neither knoweth him: but ye know him; for he dwelleth with you, and shall be in you." -- John 14:17 "But the Comforter, which is the Holy Ghost, whom the Father will send in my name, he shall teach you all things, and bring all things to your remembrance, whatsoever I have said unto you." -- John 14:26 "But when the Comforter is come, whom I will send unto you from the Father, even the Spirit of truth, which proceedeth from the Father, he shall testify of me" -- John 15:26 "Howbeit when he, the Spirit of truth, is come, he will guide you into all truth: for he shall not speak of himself; but whatsoever he shall hear, that shall he speak: and he will shew you things to come." -- John 16:13 "But God hath revealed them unto us by his Spirit: for the Spirit searcheth all things, yea, the deep things of God." -- 1 Co 2:10
Your church places so many extra-Biblical rhinestones on the wooden cross of Christ it's no wonder God's word is so obscured.
So that's all that Jesus Christ left us in the Scriptures? A riddle? A mystery to be solved by some and misunderstood by others?
Sorry. My Redeemer is far more provident and caring than that. He left us an authoritative guardian in the form of His Church, whose teaching on the true meaning of Scripture is there for any who wish to avail themsleves of it. He didn't just say "go figure".
And if the Catholic Church isn't right, then who is? You? Benny Hinn? Ken Copeland? The Methodists? The black dude in the storefront down the mall? I don't know of anybody who thinks their understanding of Scripture is incorrect. Everyone believes that their own meaning is the true one.
History has shown us clearly over the past 400 years that your recipe for understanding Scripture simply leads to a confused proliferation of customized churches. Clearly , the Bible doesn't interpret itself. That is utter nonsense.
Jesus Christ is not a God of confusion. He didn't create a dozen, two dozen or two thousand Churches. He created one. God is one and there is one Church based on one truth.
We ought to meditate on Jesus' Name, i.e. Word of God
But he answered and said, It is written, Man shall not live by bread alone, but by every word that proceedeth out of the mouth of God. - Matt 4:4
In the beginning was the Word, and the Word was with God, and the Word was God. - John 1:1
And he [was] clothed with a vesture dipped in blood: and his name is called The Word of God. - Revelation 19:13