Why Hindu Gods are always in the standing position in a Hindu temple?

Why Hindu Gods are always in the standing position in a Hindu temple?

Good question indeed. You have detective eyes. People generally doesn't notice how and in which form the Gods are in the Temple. Yes, there are definite reasons for this. So, let's start the explanation.

How the statues will be made, in which condition they will be?

Sitting Positioned:

Griha-devtas & Shanti devtas: The Gods which are worshipped as Griha-devta must be in sitting condition. Griha-devta (the deities who are kept at home) are Lakshmi, Saraswati and Ganesh.

[Many people establish many statues at home, but it is not correct. Griha-devatas will only include these three only. Other Gods like for keeping Vishnu/Shaligram, Rukmini-Kant or Gopal Krishna and Shiva/Shiva-lingam one must do daily duties like giving them bath, in case you are not at home you need to take them with you or give someone the duty of their daily care.

The statues of deities of power like Kali, Durga and others must not be at home. Temples are the perfect place for their worship. But in case of the statue of Devi Parvati, you can keep her at home. Navgraha can never be at home.]

The deities Lakshmi, Saraswati and Ganesh are shanti devtas. As the symbolize Wealth, Knowledge and Fortune. And you should give the deity a sit as she/he is in your home. Kind of mannerism. Again, they are Shanti Devtas, anywhere you worship them, they must be given sit.

In case of Devi Parvati sitting position is preferred. Shiva when worshipped in his peaceful form must be in sitting condition .

Graha Davata:

Must be in sitting condition, but on their chariot.

In case with Vahana, all will be sitting.

Likewise, Gods that will be in sitting condition are-

1. Lakshmi
2. Saraswati
3. Ganesh
4. Shiva in his soumya/peaceful form
5. All the 9 Graha Devata
6. Kuber and his wife
7. Rama & Sita in King and Queen form
8. Indra with Elephant or throne must be on them
9. Vishnu with Lakshmi
10. Jagdamba / Jagatdhatri
11. Mansa
12. Brahma
13. Rukmini-Kant on swing
14. Krishna as Gopal
15. 12 Adityas
16. Lakshmi-Nrisingha (As the are the symbol of love here, i.e. peace)
17. Gayitri (Mother, so always you need to give her seat. And she is the symbol of peace too)
18. Kamakshya
19. Tripura Sundari (as she is a symbol of a little girl, peaceful and always smilin) and many more.

Standing Positioned:

All the forms of Shakti and Gods related to power must be standing. Durga, Kali and all the forms of her must be in standing position. 11 Rudras ( Shiva in his furious form ) will always be standing. Vishnu alone will always be standing. Rukmini-Kant(Krishna), Rama,Sita and Lakshmana as sanyasi, Jagannath,will be in standing condition.

Lying Position:

Only Shiva with Kamakshya / Kali and Dev Padmanabha will be in lying position. No other God will be in this condition.

The gods representing the form of combined power like - Hara-Gauri and Hari-Har are mostly worshiped in standing posture. There is no specific mention found. You can even worship them in sitting posture. But better standing, as the tradition going on.

Many Gods like Kartikeya, 8 Vasus, Vishwakarma can be in any condition.

Statues depicting special incidents mustbe asmentioned inthe texts, as in Devi Durga is mentioned in Markendeya Purana to be keeping one leg on lion and other on Mahishasur and Mahishasur getting out of a buffalo (Mahish) - the statue must be as mentioned.
Wherever it is, the statues will be as mentioned.

Note: If I didn't mention any God do give their names in comment. I will surely include them.

Thank You.