Why is Ramayana called the adi-kavya (first poem) and not Rig Veda? As I understand Rig Veda also consist of poems and since Rig Veda is older than Ramayana, shouldn't it be called the first poem?
Vedas and Kavyas are entirely different.
Vedas are called Apaurusheya which means authorless. They were heard. Ramayana is a kavya written by man - Valmiki. Valmiki written the story of Rama using different words, analogies, embellishments, comparisons. Hence he is called Adi Kavi.
Rigveda consists of mantras. Verses in Vedas are called mantras. These are written in different meters. In Ramayana the verses are called shlokas (born from the shoka of Valmiki). A verse/poem is a blanket term used for a rhythmic/systematic arrangement which can be sung . But the words verse and poem also have a different meaning.
Just like there are features of Puranas, Kavya also has some features. Not every poem written by human author becomes a kavya. There are certain features to be fulfilled to become a poem.
For instance, according to Mammata Bhatta's Kāvyaprakāśa, definition of Kāvya (poetry) is:
tadadoṣau śabdārthau saguṇāvanalaṅkṛtī punaḥ kvāpi
A combination of word and meaning, which is free of blemishes, endowed with qualities, alamkaras and sometimes the poetic figure is absent.
So which contains these features will be called a Kāvyaṃ. More definitions and information on a poem are found in relevant page on wisdomlib.org
There are several grammarians who gave their definition of Kāvya (poem) and some criticized Mammata's definition. However it's clear that a poem has different set of features. It contains embellishments, analogies, rhyming according to a proper meter etc., Rigveda on the other hand is not for reading with analogies, embellishments etc., there are seers who head the mantras due to a deep meditation technique.
Ramayana is the oldest literary work to be written with above features. Hence it is called Adi Kavya.
Shodhganga's article What is Kavya?, explains
Imaginations and emotions are basic concepts of Kavya (creative literature) which are often expressed in metrical compositions. Kavya has figurative language, typical diction, nature, and words in various combinations, which means the choice and arrangement of words and their structure. Kavya combines pleasure with profit. A poet uses words to produce images.
Kavya excels in appealing to listeners in broader details. Kavya has special characteristics. It includes all creative literature such as prose and verse. It is an imaginative representation of life. Imagination and emotion are basic concepts of Kavya. The emotional interpretation of life is presented in rhythmical language.
Both Rig Vedic mantras and EPICs contain poems. The basic differences, in my view, between the Hymns in Rig Veda,say Indra's eliminating Vritra, Saraswati Suktam, etc, and EPICs like Ramayana, are that
- the Rig Vedic Hymns eulogise the Almighty God in esoteric/cryptic manner - understandable to spiritually elevated persons, whereas the EPIC like Ramayana is understandable to a common man also.
- the poems in EPICs like Ramayana were composed in Classic Sanskrit, whereas that of Rig Veda were composed in Vedic Sanskrit.
- The poems in Rig Veda may contain references to some historical figures like Usana and do not contain any specific story except eulogising the Almighty in crypic manner, but the EPIC is all about the story of a central character with underlying theme.
- In Ramayana the central character is Sri Rama, whereas the central Character of Mahabharata is Yudhisthira. In both the EPICs the underlying theme is DHARMA. Sri Rama and Yudhisthira were described by respective poets as dharmAtma - embodiment of Dharma (rAmO vigrahavAn dharmaha).
- the emotions that ordinary human beings carry, are depicted in a kaavya like Ramayana.
As the kaavya (EPIC) Ramayana, containing the story of Sri Rama, was composed first, it is called Adi kAvya.