I'm not sure in exactly what sense you don't think it isn't like the Constitution. It's not a Political document, sure, but it was written by various men at different times and collected by other men who VOTED on what should be included an what shouldn't. So it has been amended, and it contains all sorts of stuff that is ignored by its readers simply becasue they don't want to cut it out, but they don't use it. Wildly contradictory in places, too, but that's generally ignored as well. I dare say it could stand some ammending, if you're not ready to throw it out altogether (I'm ready to throw it out altogether).
Anyway, on to the Pope. The Catholic Church doesn't hold that all revelation ended with the last book in the Bible. The Church doesn't add new books, but it does believe that Revelation continues. This is why there is a belief in Saints, and in appearances by same as well as by Mary.
The Church has a council of Cardinals and Bishops, I think they call the Cardinals a "college," who discuss Church matters, as well as how the Catholic Church, as a Political force, ought to be using its influence on world events. The Pope is the head Cardinal. So while he can't amend or add to the Bible, his religious power is obvious.
You said something elsewhere about how in Catholicism you can "apologise for anything anyway." I suppose you were referring to Confession, which Catholics use to air out their mistakes and receive moral guidance through prayer. I suppose you're some other form of Christian, and so you think this is silly. Power to you. I don't believe it's any sillier than anything else in Christianity. A being who is ONE and is also his own son, as well as the love between the father and son which is manifested sometimes as a bird that can make people talk in different languages, but which is unique although the three are all actually only ONE thing, created the world and everything in it, either animals first and then man and woman, or animals and man, and then woman, or something. Then this being got mad and said everyone was a sinner, but they could be forgiven if it came to earth in the form of the son and let the poeple kill it so that it could come back to life and then tell them "you're forgiven just so long as you believe that all of this happened." And the primary source for this was some books that were lauded around the Roman Empire 2000 years ago.
It's all nonsense.
Just like Lennon said, and Benedict apparently agrees, "I don't believe in Zimmerman; I just believe in me."