Why does using ConvertDocument not directly set pageSetup, but require secondary processing of the page

[Sunny-UU]

const wordsApi = new WordsApi(clientId, secret);
const document = Readable.from(content)

const request = new ConvertDocumentRequest({
  format: "docx",
  document,
});

//first processing of convertDocument
const word = await wordsApi.convertDocument(request)

const wordBuf = Readable.from(word.body)

const requestPageSetup = new PageSetup({
  paperSize: PageSetup.PaperSizeEnum.A4,
  orientation: PageSetup.OrientationEnum.Landscape
})

//secondary processing of the page
const updateRequest = new UpdateSectionPageSetupOnlineRequest({
  document: wordBuf,
  sectionIndex: 0,
  pageSetup: requestPageSetup
});

const wordsResponse = await wordsApi.updateSectionPageSetupOnline(updateRequest)

fs.writeFile("a4.docx", wordsResponse.body.document.values().next().value);