The avatara or incarnation concept is a relatively new one to Sanaatana Dharma, otherwise known as Hinduism.
If we remove the interpolations and read Srimad Ramayana and Mahabharata, which closely followed the Vedic way of life, we will understand that there is no incarnation concept in those epics.
We will find the description of Vishnu taking the form of Vamana, a dwarf, for overcoming Bali, as per the wishes of Gods, in Srimad Ramayana, but there is no incarnation.
बलिः वैरोचनिः विष्णो यजते यज्ञम् उत्तमम् |
असमाप्त व्रते तस्मिन् स्व कार्यम् अभिपद्यताम् || १-२९-७
Oh, Vishnu, Bali the son of Virochana is conducting an unsurpassed Vedic ritual, before the completion of which our own mission, say the task of gods, is to be achieved completely.
स त्वम् सुर हितार्थाय माया योगम् उपाश्रितः |
वामनत्वम् गतो विष्णो कुरु कल्याणम् उत्तमम् || १-२९-९
'As such, oh, Vishnu, you may effectuate the most suitable merited deed for the welfare of gods on resorting to your illusory power, and on assuming the form of a dwarf to deal with Emperor Bali.' Thus gods told to Vishnu.
It was not Lord Vishnu, who took the Varaha Avatar, but Lord Brahma. Srimad Ramayana clearly states this.
सर्वम् सलिलम् एव आसीत् पृथिवी यत्र निर्मिता |
ततः समभवद् ब्रह्मा स्वयम्भूर् दैवतैः सह || २-११०-३
All was water only in the beginning" from which element the earth was formed. After that, the self-existent Brahma with all the gods came into existence.
स वराहः ततो भूत्वा प्रोज्जहार वसुंधराम् |
असृजच् च जगत् सर्वम् सह पुत्रैः कृत आत्मभिः || २-११०-४
"Thereafter, that Brahma, assuming the form of boar, caused the earth to rise from water and with his sons of pure soul, created the entire world."
The incarnation concept was started with the Puranas, which came at a much later period.
