Since I participate on some blogs run by Protestants, I run into the “point to it in Scripture or shut up” mentality now and then. I don’t figure those blogs are the appropriate place to argue that point, though. So here’s my simple understanding of why Catholics don’t do Sola Scriptura (Scripture alone).
Our goal is always to follow the teachings of Jesus Christ, because He is, after all, the living Word of God. He passed His teachings down to His apostles, who passed them to their disciples, and so on. As the Church grew, the Apostles appointed bishops to carry their authority to new cities and nations. After a while, they started to realize He wasn’t coming back quite as soon as they originally thought, so they began to write down the story of His time on earth and some of His teachings. But as John tells us, these were only the highlights:
But there are also many other things which Jesus did; were every one of them to be written, I suppose that the world itself could not contain the books that would be written. — John 21: 25
So we don’t expect all of God’s Word to be in Scripture. Scripture itself tells us it’s not the whole story. So our primary source remains Apostolic Tradition: the teachings as passed from Jesus to the Apostles to their disciples and down through the episcopal line of succession to today’s bishops. (Our bishops are far from perfect, but like them or not, they’re still our link to the Apostles.) We also have the Scriptures, again detailing some of Jesus’s teachings, which have been compiled, translated, approved, and interpreted by that same line of authorities. Without the Church and her dedication to preserving Christ’s teaching, the Scriptures never would have survived long enough to be translated by anyone else.
So we see the Scriptures as part of Tradition — a very special part to be sure, since the Church has declared them to be divinely inspired Truth — but still only a part, and not surpassing all the rest.
Another part of this is the issue of interpretation. Look how many different meanings people can get out of the US Constitution, a much shorter document written in English only a bit over two centuries ago. Extrapolate that to a collection of documents written in Greek and Hebrew 2000+ years ago and translated into the vernacular, generally not from the original sources, and written in the context of a society very different from ours. Unless I’m a biblical scholar, what are the chances that I, studying at home alone, will always find the correct interpretation? Yes, I can expect guidance from the Holy Ghost, but I can’t assume that that guidance will always overcome my own ignorance and biases.
So we look to the Church — again to the authority of the bishops passed down from Jesus through the Apostles — to tell us what the Scriptures mean. That doesn’t mean we shouldn’t read Scripture and have study groups and try to gain a deeper understanding of our own. We absolutely should. But we need a guide along the path to understanding, and we need some reason to trust that guide. I trust my guide — my parish priest, or the study bible I use that has the Imprimatur — because it speaks with the authority of the Church Christ founded.
Edit: Novaseeker had a similar inspiration to write on the same topic. Check it out; he goes over the history of Sola Scriptura and covers the problems with it more thoroughly than I did. He uses cool big words like hermeneutics, too.