Se você ler a promessa com cuidado, há uma grande falha no presente solicitado por Percy, que foi usado pelos Deuses para salvar: Não havia linha de tempo para perdoar os Titãs como Calypso.

Percy queria que todos os semideuses fossem reivindicados antes de completar 13 anos e os Deuses cumpriram sua promessa.

“Very well!” Zeus growled. “In the name of the Council, we swear by the River Styx to grant your reasonable request as long as it is within our power.”

The other gods muttered assent. Thunder boomed, shaking the throne room. The deal was made.

“From now on, I want to you properly recognize the children of the gods,” I said. “All the children . . . of all the gods.”

The Olympians shifted uncomfortably.

“Percy,” my father said, “what exactly do you mean?”

“Kronos couldn’t have risen if it hadn’t been for a lot of demigods who felt abandoned by their parents,” I said. “They felt angry, resentful, and unloved, and they had a good reason.”

Zeus’s royal nostrils flared. “You dare accuse—”

“No more undetermined children,” I said. “I want you to promise to claim your children—all your demigod children—by the time they turn thirteen. They won’t be left out in the world on their own at the mercy of monsters. I want them claimed and brought to camp so they can be trained right, and survive.”

“Now, wait just a moment,” Apollo said, but I was on a roll.

“And the minor gods,” I said. “Nemesis, Hecate, Morpheus, Janus, Hebe—they all deserve a general amnesty and a place at Camp Half-Blood. Their children shouldn’t be ignored. Calypso and the other peaceful Titan-kind should be pardoned too. And Hades—”

_{^{The Last Olympian, Chapter 20, We Win Fabulous Prizes}}

Grover menciona em The Lightning Thief, que os Três Grandes já haviam feito uma promessa no Rio Styx.

Grover shifted his hooves uncomfortably. "About sixty years ago, after World War II, the Big Three agreed they wouldn't sire any more heroes. Their children were just too powerful. They were affecting the course of human events too much, causing too much carnage. World War II, you know, that was basically a fight between the sons of Zeus and Poseidon on one side, and the sons of Hades on the other. The winning side, Zeus and Poseidon, made Hades swear an oath with them: no more affairs with mortal women. They all swore on the River Styx."

_{^{The Lightning Thief, Chapter 8, We Capture A Flag}}

E dois deles não conseguiram mantê-lo, mas esses deuses " saíram fácil ".

Grover's face darkened. "Seventeen years ago, Zeus fell off the wagon. There was this TV starlet with a big fluffy eighties hairdo—he just couldn't help himself. When their child was born, a little girl named Thalia .. . well, the River Styx is serious about promises. Zeus himself got off easy because he's immortal, but he brought a terrible fate on his daughter."

_{^{The Lightning Thief, Chapter 8, We Capture A Flag}}

Assim, mesmo que Percy tivesse especificado o prazo dentro do qual Calypso seria libertado, os Deuses poderiam ter socorrido com leve punição.

Eles se esqueceram e ninguém se certificou de que cumprissem a promessa.

Como @Vishvesh mencionou na sua excelente resposta , os deuses são fáceis porque são imortais:

Grover's face darkened. "Seventeen years ago, Zeus fell off the wagon. There was this TV starlet with a big fluffy eighties hairdo—he just couldn't help himself. When their child was born, a little girl named Thalia .. . well, the River Styx is serious about promises. Zeus himself got off easy because he's immortal, but he brought a terrible fate on his daughter."
_{Percy Jackson & the Olympians, book 1: The Lightning Thief, chapter 8: "We Capture A Flag"}

Mas Percy menciona em O Sangue do Olimpo que ele não checou sobre eles, e os deuses tendem a esquecer as coisas (eu tenho certeza que há uma citação sobre isso em algum lugar, mas eu não pode puxá-lo no momento por algum motivo).

"Hey," Percy said. "If we don't make it out of this..."
"Shut up, man. We're going to make it."
"If we don't, I want you to know - I feel bad about Calypso. I failed her."
Leo stared at him, dumbfounded. "You know about me and -"
"The Argo II is a small ship," Percy grimaced. "Word got around. I just... well, when I was in Tartarus, I was reminded that I hadn't followed through on my promise to Calypso. I asked the gods to free her and then... I just assumed they would. With me getting amnesia and getting sent to Camp Jupiter and all, I didn't think about Calypso much after that. I'm not making excuses. I should have made sure the gods kept their promise. Anyway, I'm glad you found her. You promised to find a way back to her, and I just wanted to say, if we do survive all this, I'll do anything I can to help you. That's a promise I will keep."
_{The Heroes of Olympus, book 5: The Blood of Olympus, chapter 12}

... que é um discurso bastante longo, dadas as circunstâncias, mas seja o que for. O ponto é que os deuses simplesmente não cumpriram sua promessa - e ninguém jamais pensou em checá-los.

Eu gosto da teoria do limite de tempo. Eu gostaria de acrescentar que, eventualmente, deixá-la sair, enviando Leo lá duas vezes. Ninguém ia lá duas vezes, mas ele o fez, o que obviamente era um Deus que dobra as regras para cumprir a promessa. Eu sinto que será Hefesto enquanto ele visitava Calypso regularmente e era o pai de Leo.