The Lord says in Bhagavad-gītā (4.6) :

ajo ’pi sann avyayātmā bhūtānām īśvaro ’pi san prakṛtiṁ svām adhiṣṭhāya sambhavāmy ātma-māyayā

“Although I am unborn and My transcendental body never deteriorates, and although I am the Lord of all sentient beings, I still appear in every millennium in My original transcendental form.” In the following verse in Bhagavad-gītā (4.7) , the Lord says:

yadā yadā hi dharmasya glānir bhavati bhārata abhyutthānam adharmasya tadātmānaṁ sṛjāmy aham

“Whenever and wherever there is a decline in religious practice, O descendant of Bharata, and a predominant rise of irreligion — at that time I descend Myself.” Thus although the Supreme Lord is unborn, there is no cessation to His appearance in different forms as incarnations like Lord Kṛṣṇa and Lord Rāma. Since His incarnations are eternal, the various activities performed by these incarnations are also eternal. The Supreme Personality of Godhead does not appear because He is forced to do so by karma like ordinary living entities who are forced to accept a certain type of body. It is to be understood that the Lord’s body and activities are all transcendental, being free from the contamination of the material modes of nature. These pastimes are transcendental bliss to the Lord. The word aparigaṇya-dhāmne is very significant. There is no limit to the Lord’s appearance in different incarnations. All of these incarnations are eternal, blissful and full of knowledge.