As I discuss in this answer, how the Vedas originated is that from time immemorial, sages known as Dhristas (literally seers) have heard sacred verses (mantras) directly from the gods during a state of Tapasya (deep meditation). And then in the Dwapara Yuga (the age before the current one), a sage named Krishna Dwaipayana Vedavyasa (Vyasa 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 Vyasa only compiled the Rig, Yajur, and Sama Veda; the Atharvana Veda was compiled by the sages Atharva and Angiras).
But Vyasa wasn't the first one to compile and divide the Vedas, and he won't be the last. Vedavyasa is in fact a title, assigned to a different person in every Dwapara Yuga, as described by Vyasa's father Parashara in the Vishnu Purana:
Twenty-eight times have the Vedas been arranged by the great Rishis in the Vaivaswata Manwantara in the Dwápara age, and consequently eight and twenty Vyásas have passed away; by whom, in their respective periods, the Veda has been divided into four. In the first Dwápara age the distribution was made by Swayambhu (Brahmá) himself; ... I [Parashara] was the Vyása of the twenty-sixth Dwápara, and was succeeded by Jaratkáru; the Vyása of the twenty-eighth, who followed him, was Krishńa Dwaipáyana. These are the twenty-eight elder Vyásas, by whom, in the preceding Dwápara ages, the Veda has been divided into four. In the next Dwápara, [Ashwatthama] Drauńi (the son of Drońa) will be the Vyása, when my son, the Muni Krishńa Dwaipáyana, who is the actual Vyása, shall cease to be (in that character)
My question is, why do the Vedas need to be recompiled and redivided in every Dwapara Yuga? I can understand if the knowledge of the Vedas is lost over the generations as people die, and thus in every Dwapara 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 Chiranjeevis (immortals), so why doesn't the new Vedavyasa just ask some previous one for their compiled work?
Vyasa's father Parashara is immortal, for instance, so couldn't Vyasa have just asked his dad for the verses he had compiled two Dwapara Yugas ago? And Ashwatthama is immortal (as I discuss in this answer), and he's alive right now, so rather than waiting until the next Dwapara Yuga, couldn't he just walk into a bookstore tomorrow and buy a copy of Vyasa's version of the Vedas? Or if there's an issue of all the original Shakhas (recensions) not being included in the books they sell in bookstores, couldn't Ashwatthama have just gotten the Vedas from Vyasa himself in the last Dwapara Yuga, especially considering that he was a student of Vyasa?
Is the issue simply that the compiled Vedas grow in size each time, because in the intervening years between Dwapara Yugas, sages will have heard certain verses from the gods that had never been heard before? Is Vyasa's version of the Vedas bigger than Parashara's, and will Ashwatthama's be even bigger? Are there any scriptures that describe this?