As I discuss in this answer, how the Vedas originated is that from time immemorial, sages known as Dṛṣṭas (literally seers) have heard sacred verses (mantras) directly from the gods during a state of Tapasya (deep meditation). And then in the Dvāpara Yuga (the age before the current one), a sage named Kṛṣṇa Dvaipāyana Vedavyāsa (Vyāsa for short) compiled these verses from all those different sages into a coherent work, dividing them into a set of four books we call the Vedas. (Technically Vyāsa only compiled the Rig, Yajus, and Sāma Veda; the Atharvaṇa Veda was compiled by the sages Atharva and Aṅgiras).
But Vyāsa wasn't the first one to compile and divide the Vedas, and he won't be the last. Vedavyāsa is in fact a title, assigned to a different person in every Dvāpara Yuga, as described by Vyāsa's father Parāśara in the Viṣṇu purāṇa:
Twenty-eight times have the Vedas been arranged by the great Rishis in the Vaivaśvata Manvantara in the Dvāpara age, and consequently eight and twenty Vyāsa have passed away; by whom, in their respective periods, the Veda has been divided into four. In the first Dvāpara age the distribution was made by Svayambhu (Brahma) himself; ... I [Parāśara] was the Vyāsa of the twenty-sixth Dvāpara, and was succeeded by Jaratkāru; the Vyāsa of the twenty-eighth, who followed him, was Kṛṣṇa Dvaipāyana. These are the twenty-eight elder Vyāsas, by whom, in the preceding Dvāpara ages, the Veda has been divided into four. In the next Dvāpara, [Aśvatthāma] Drauṇi (the son of Droṇa) will be the Vyāsa, when my son, the Muni Kṛṣṇa Dvaipāyana, who is the actual Vyāsa, shall cease to be (in that character).
My question is, why do the Vedas need to be recompiled and redivided in every Dvāpara Yuga? I can understand if the knowledge of the Vedas is lost over the generations as people die, and thus in every Dvāpara Yuga someone needs to go back to all the sages and find out the verses they heard from the gods. But most of the people mentioned in the above quote are Cirañjīvis (immortals), so why doesn't the new Vedavyāsa just ask some previous one for their compiled work?
Vyāsa's father Parāśara is immortal, for instance, so couldn't Vyāsa have just asked his dad for the verses he had compiled two Dvāpara Yugas ago? And Aśvatthāma is immortal (as I discuss in this answer), and he's alive right now, so rather than waiting until the next Dvāpara Yuga, couldn't he just walk into a bookstore tomorrow and buy a copy of Vyāsa's version of the Vedas? Or if there's an issue of all the original Śākhās (recensions) not being included in the books they sell in bookstores, couldn't Aśvatthāma have just gotten the Vedas from Vyāsa himself in the last Dvāpara Yuga, especially considering that he was a student of Vyāsa?
Is the issue simply that the compiled Vedas grow in size each time, because in the intervening years between Dvāpara Yugas, sages will have heard certain verses from the gods that had never been heard before? Is Vyāsa's version of the Vedas bigger than Parāśara's, and will Aśvatthāma's be even bigger? Are there any scriptures that describe this?