> "couldn't they just fly to Mount Doom and drop the ring?"
If the allies were counterfactually sensible enough to fly the ring to Mordor, Sauron could have been counterfactually sensible enough to station an Orc/Troll Battlegroup at the Sammath Naur, with a Nazgul combat air patrol.
Note I wasn't really trying to go into the argument, just pointing out these are well-known and very debated topics in Tolkien fandom.
My own opinion is that debating this is missing the point. Tolkien was about the hero's journey, which necessitates the hard path to victory. It's not at all about flying a modern superweapon into Mount Doom; that's too literal a reading.
If trying to rationalize things - I'd say Sauron knows that giant eagles are a thing, and able to serve as mounts. So to prevent Western aerial reconnaissance and insertion/extraction of observers/spies/special forces in Mordor, he's got to have some sort of aerial observer / aerial denial systems going. Which systems would make a "fly the Ring to the fire" gambit too risky.
(Vs. voice-of-canon Gandalf makes it clear that anyone seeking to destroy his Preciousss is simply beyond Sauron's Vile McEvil worldview.)