One of the main process of yajna is to prepare food to be offered to a particular deity which is known as havi. I have read somewhere that in order to maintain balance as being immortals the Gods can not make their food.
That brings humans into picture as they offer food in yajna, and since this is a reciprocal relatinship, Gods in turn give humans the riches of cattle, food and sons.
The reciprocal relationship stems from the following verse of Rigveda which states Indra (also Indra has an epithet 'yajñávr̥ddha') :
aháṃ dāṃ gr̥ṇaté pū́rvyaṃ vásu, ahám bráhma kr̥ṇavam máhyaṃ várdhanam (X, 49, 1) 'I shall give to the singer the ancient boon, I shall make prayer the means of growth for me'.
Also, the food depends on the deity to be pleased. For example Pusan loves porridge since Rudra broke his teeth in one of their altercation hence barley porridge is offered to him in yajna.
My question is that since rigveda tells about this reciprocal relationship does later texts provide any account why Gods can not make their own food?