ignoring all the in-universe reasons like "the eagles were established to be aloof" and "sauron would see that coming with his big eye" and all that jazz
the real reason is that it would make for fucking awful story
The real reason is Christianity. Basically, because of stuff from earlier in history, Middle Earth is basically "fallen," though "god" is still active there in some way, "god" (Illuvatar) and the "angels" (the Valar) are in some ways cut off from the mortals. The Eagles are the servants of Manwe, the chief Valar. Like in (some versions of) Christian theology, God doesn't just save you; you have to try your best to do all that you're supposed to do (follow the commandments, receive the sacraments, etc.). This is not enough though, you still need grace to be saved, there's some verse I kinda remember that's like "it is by grace we are saved, after all that we can do." Middle Earth has to deal with the problem of Sauron. Were the Eagles actually willing to just fly them to Mordor, it wouldn't have even reached that point cuz they could have just shipped the ring over the ocean to Valinor, or the Valar could have just invaded Mordor themselves. And Frodo needs to try his hardest to achieve the task appointed to him. Of course, none of this would have been possible without some guidance from "god" (Gandalf is basically an angel, for example). And even at the end, Frodo's mission would have failed had not Illuvatar intervened to make Gollum fall into the fire, and also Frodo would have died in the eruption afterwards. But because him and Sam endured to the end to the best of their ability, they get grace and are saved by the Eagles.
Wow, I came here to say this.
And for a supposedly hands-off Creator, Iluvatar sure gets involved a lot. Not just at Mount Doom, but he raised Gandalf from the dead and did a bunch of other stuff that I forget.
GOLLUM
WAS
PUSHED
Not to mention, the name "Elbereth" saves them from the ringwraits I think twice that I remember? The Valar and Illuvatar were definitely doing some shit lol
This is not enough though, you still need grace to be saved, there's some verse I kinda remember that's like
For by grace are ye saved through faith, and that not of yourselves, it is the gift of God. Not of works, lest any man should boast.
I really fucking hate that I wasted so many hours of my childhood in AWANA and this is the shit that sticks in my brain while I have a hard time remembering any meaningful definition of dialectical materialism.
I was a Meritorious Award winner, and on the off chance anyone reading this knows what that means you will have some idea of how much of goddamn nerd I am.
the real reason is that the eagles would get absolutely annihilated by nazgul and everyone would die and sauron would win. the entire point of frodo's journey was to sneak the ring into mordor under sauron's nose because neither the fellowship nor any of their allies had the strength to fight their way into mordor. the eagles are not sneaky.
And a big reason that Frodo and Sam were able to get in was because Sauron and all his military strength was distracted by the Battle of Gondor. Or more easily,
"Shut up." - J.R.R. Tolkien
