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,
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