system
Given the following post, respond as if you're commenting on Reddit:
human
[LOTR] Why didn't Sauron build all of his fortifications around Mt Doom?

If the only thing that could destroy the ring of power was throwing it into Mt Doom, why is there no army/fortifications/even one guy with a sword keeping on eye on all that lava? Especially since, according to the film, there's only one entrance! One orc standing in the doorway would have killed Sam and Frodo easily!

Edit: thanks so much for the answers! So the consensus seems to be that Sauron couldn't spare one orc for mount doom... And assumption is the mother of all fuck ups!

But in all seriousness, thank you for the detailed responses, I guess I hadn't thought of how certain Sauron would be that the ring would look after itself.
gpt
Your question is answered multiple times in the books, and even addressed in the films.

Sauron, like everyone else in the world, views things through his own lens and with his own biases.  As he desires power and dominance, so he assumes all his foes do.  That anyone might throw away a weapon like the One Ring is a thought that would never enter his mind.  Until the instant of his demise he never once considered the possibility that someone might choose to unmake the Ring and, in any event, the Ring's own mind-warping effects made that choice impossible.  Only the unexpected appearance of Gollum at the end allowed the Ring to be destroyed, even Frodo and Isildur, when given the chance, couldn't willingly throw the Ring into the fires.  Defending his long abandoned forge would be a waste of Sauron's resources.  

Sauron believed the Free Peoples had surrendered the Ring to their strongest leader, the Heir of Gondor.  He genuinely believed the Ring was with Aragorn at Minas Tirith awaiting the new King's coronation to claim it as his own.

That is why Aragorn was able to call out Sauron's forces from Mordor at the Battle of the Black Gate.  Sauron was trying to recover the Ring and simultaneously crush all opposition.  Victory there would be final victory for the Mordor.  That, in turn, led to most of Mordor being cleared of troops, greatly reducing Sam and Frodo's troubles.