Of Eagles and, well, eagles...

[Enedbelethiel]

This is actually a newbie kind of question, but I figured it fit better under this category since it has to do with the RoTK movie. In the end when the eagles swoop down to pick up Sam and Frodo from Mordor, I couldn't help but wonder why the eagles couldn't just pick them up from Rivendell in the first place and take them straight to Mordor. I guess that obviously would have killed the plot of the story but I still couldn't help thinking about it. In fact, I could ask this same question from the book sense, too. The eagles actually seemed invincible to orc forces, and I doubt that any of the fell beasts could have done much damage to them. Anyone with an answer?