Eagle-eyed

Just finished watching Return Of The King on the telly, first time I've seen it since I saw it at the cinema. And then too I didn't watch it fully, as I dozed off a third of the time! Anyway, watching the ending again, I'm compelled to ask the following:

If Gandalf can use the giant eagles to go rescue Frodo and Sam from Mount Doom, if it seems so easy, why not just use the same damn eagles to send them there in the first place? Would save a lot of humans, elves, dwarves and whatever creature from dying, me reckons.

All you Lord Of The Rings fans, I expect your answer!