The Horde Breaker Ranger feature's descriptions clearly says:
you can make another attack with the same weapon against a different creature that is in within 5 feet of the original target
If you fight a giant creature, say Tiamat, wouldn't it make sense you could target two distinct parts of her body using this feature?
Is this possible? If not, why?
Horde Breaker does exactly what it says it does. You can target a different creature. Not the same one, no matter how big the creature is.
You're asking, "If I can shoot two arrows five feet apart against two goblins, why can't I shoot an ogre twice, five feet apart?" Well, you can't because the special ability that you took doesn't give you that option. The rule is what the rule is, and whether it "makes sense" isn't, frankly, part of the discussion.
Okay, so why not? Why doesn't "makes sense" play into this?
First off, these kind of abilities are often more narrative than physical. It's not about being able to place X number of strikes within Y feet, it's the story being told. A ranger who takes Horde Breaker is doing what the title suggests, they've trained to be able to fight large numbers of weak enemies effectively, not how to turn a dragon into a pincushion.
Second, there's a balance concern. If you want to take down giants, there's literally an ability for that. Consider a comparison: Colossus Slayer lets you deal extra damage once per round (provided you make a hit and the target's already been damaged once, which is pretty much always for big monsters with lots of HP). Giant Killer lets you make one extra attack per round (provided you're up close and they attack you). Horde Breaker lets you make one extra attack per round (provided you have a second nearby target to shoot).
If you could use Horde Breaker to hit the same large target one extra time per round, then it would be vastly better than Giant Killer: you still get an extra attack per round, but you don't need to be up close, you don't need the monster to attack you, and you can hit two different targets when you aren't fighting a single big enemy. It's basically Horde Breaker when facing hordes and Giant Killer when facing giants.
It would make Horde Breaker better than Colossus Slayer for similar reasons; when facing a monster with lots of HP you can hit it twice, and when facing weak monsters that die from one hit, you can shoot two at a time.
If your house rule would make one mechanical option strictly better than every other option, then it's a bad rule.
