Why aren't dinosaurs mentioned in the Hindu mythology?

The description of Dinosaurs as was made in Ramayana may not be the same that we hear now-a-days , but a definite mention was made about a monstrous aqua creature - Simhika - in Sundara Kanda, Ramayana, wherein a fight between Sri Hanuma and Simhika was described.

प्लवमानं तु तं दृष्ट्वा सिंहिका नाम राक्षसी |

मनसा चिन्तयामास प्रवृद्धा कामरूपिणी || ५-१-१८२

A demoness by the name of Simhika with the power to assume desired form, saw Hanuma flying, increased in size and thought thus in her mind.

अद्य दीर्ग़्हस्य कालस्य भविष्याम्यहामाशिता |

इदं हि मे महत्सत्त्वं चिरस्य वशमागतम् || ५-१-१८३

"After a long time, this great animal came into my grasp. After long time, today I will enjoy my meal."

इति संचिन्त्य मनसा छायामस्य समाक्षिपत् |

छायायां गृह्यमाणायां चिन्तयामास वानरः || ५-१-१८४

Thinking thus in her mind, Simhika attracted His shadow. While the shadow was thus being grasped, Hanuma thought thus.

समाक्षिप्तोऽस्मि तरसा पञूकृतपराक्रमः |

प्रतिलोमेन वातेन महानौरिव सागरे || ५-१-१८५

"Like a great boat in the ocean with opposing winds, I am being pulled back forcefully, with disabled power."

तिर्यगूर्ध्वमधश्चैव वीक्षिमाणस्ततः कपिः |

ददर्श स महत्सत्त्वमुत्थितं लवणाम्भसि || ५-१-१८६

After that, Hanuma looking sideways, upwards and downwards, saw a big animal raised up in the salty ocean.

छाय्तद्धृष्ट्वा चिन्तयामास मारुतिर्विकृताननम् |

कपिराजेन कथितं सत्त्वमद्भुतदर्शनम् || ५-१-१८७

Hanuma saw that animal with a horrible face and thought: "This animal with a strange appearance, with great strength attracting shadow, is indeed the animal that had been told by Sugriva. There is no doubt in that."

आग्राहि महावीर्यं तदिदं नात्र संशयः |

स तां बुद्ध्वार्थतत्त्वेन सिंहिकां मतिमान्कपिः |

व्यवर्धत महाकायः प्रवृषीव वलाहकः || ५-१-१८८

That wise Hanuma recognizing that animal correctly as Simhika, increased His body greatly, like a cloud in rainy season.

तस्य सा कायमुद्वीक्ष्य वर्धमानं महाकपेः || ५-१-१८९

वक्त्रं प्रसारयामास पाताळान्तरसन्निभम् |

That Simhika seeing the growing body of the great monkey Hanuma, spread her mouth to be equal to the middle of Patala.

स तस्या विवृते वक्त्रे वज्रसंहननः कपिः |

संक्षिप्य मुहुरात्मानं निष्पपात महाबलः || ५-१-१९२

That Hanuma with great strength and with a body equal to a diamond, repeatedly contracted His body and fell into her opened mouth.

ततस्तस्या नखैस्तीक्णैर्मर्माण्युत्क ृत्य वानरः |

उत्पपाताथ वेगेन मनः संपातविक्रमः || ५-१-१९४

Then Hanuma rend her internal organs with His sharp nails and after that flew up with a speed equal to that of thought.

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Thus, in my view, that great water animal, which was killed by Sri Hanuma, must be a dinosaur.

Now a question arises whether dinosaurs lived in water?  Please go through the following material at the link provided below:

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Professor Brian J Ford said the prehistoric creatures “just don’t work” in the way palaeontologists have understood for decades.

He believes their tails were too large and cumbersome for them to hunt or move with agility, and said they could not have consumed enough food to sustain their energy needs.

Instead, he claims dinosaurs must have lived in water where their environment would support their bulk successfully.

Dinosaurs must have lived in water, scientist claims