Why is Luffy the only character that runs out of Haki?
The limits of Haki usage can vary depending on a character’s training, experience, and stamina. It’s possible for any character who uses Haki to run out of it if they overexert themselves or use it extensively in battles. Luffy, being the main protagonist of One Piece, has been shown using Haki frequently, and there have been instances where he has exhausted his Haki during intense battles.
However, this is not unique to Luffy, and other characters in the series can also run out of Haki if they push themselves too hard. In Wano everyone pretty much runs out of Haki at some point. Maybe not Kaido although he was looking kinda ragged at the end.
That said, running out of Haki is totally used as a plot point. Mostly because power scaling isn’t absolute in One Piece and SS1 Goku vs Freeza style fights are kinda boring these days. It’s no fun when one fighter absolutely dominates the other thanks to a power-up or secret technique.
Luffy always pushes his body to places where he isn’t quite ready to yet, due to his immense willpower. For example Luffy incorporates Haki into his Gear 4 so it’s always active. If it’s always active it drains away. Unlike Gear 2 where he applies it and then strikes or can stop and restart usage of it he can’t in Gear 4.
Also in Gear 5, despite the strength of this form, due to Luffy’s inexperience, it consumes plenty of energy to maintain and use, even to the point that Luffy can visibly age once de-transformed, and experience an extensive amount of exhaustion and fatigue.