Hi Stephanie
Actually I can see why they are still in use, and the logic behind it. Today when you buy a car it is designed to last between 5 - 10 years (maybe 150,000 miles), but if you look back a bit cars were made to last longer with in the very early days them being expected to last a life time and going down from there.
One of the best airlines I've ever flown was the Royal Nepalese Airline (internal in Nepal). On that expedition one of the members was a pilot who owned his own small airline, and he always went round and visually inspected any plane before he got on. His comment about the rather aging plane we got on in Nepal was that he thought it was better maintained than most planes used in transatlantic flights (and you should have heard the silence after he pronounced that little ditti).
Now I veered off point there more than a little, but the point is that the space shuttles are still in their designed lifespan. Nearly all of the components inside have been replace, and I think the only thing that is the same now is the fuselage. They are getting to the end of that span now, and I think that is why commercial ventures are being encouraged, because once industry gets involved then prices come down and 'just' getting people in to space is something NASA will not have to worry about as much. They could even charter a private flight to get their astonaughts to the ISS (if its still around, and there are good arguments that it shouldn't be... but that’s another story), and that makes it cheaper.
The technology that built the shuttle is now very old hat. I mean look at the aerodynamics of the shuttle for a start... its got the dynamics of a brick. It is time to move on... but the shuttle isn't quite past its sell by date yet.
Mike