Biographies of American presidents are among the longest books in general publishing, frequently arriving in multiple volumes. The length comes from the material rather than from the author's preference.
The record is preserved by law
Presidential papers, correspondence, schedules, memoranda and recordings are retained and eventually opened to researchers, producing an archive far larger than any private life leaves behind.
A biographer working from that material is not searching for evidence but selecting from an overwhelming quantity of it, which is a different and slower problem.
Leaving out documented material is also harder to justify to reviewers who know the archive, so the pressure runs toward inclusion.
The subject is national history
A president's decisions touch legislation, courts, the military, the economy and foreign relations, and each requires enough context for a general reader to follow.
That context is effectively a second book running alongside the life, and it cannot be cut without making the decisions unintelligible.
Biographers handle it by narrating events through the president's desk, but the material still occupies a large share of the pages.
Readers of the category expect scale
The audience for these books is unusually committed, often reading several biographies of the same figure, and it treats length as a sign of seriousness.
A short presidential biography reads as an introduction, and the market for introductions is well served by existing series.
Publishers therefore acquire the long version, since that is what distinguishes a new book in a crowded field.
Volumes solve a physical problem
Beyond a certain page count a book becomes difficult to bind, expensive to print and unpleasant to hold, which forces a split into volumes.
Splitting also lets a publisher recover money earlier and lets an author publish while continuing to research the later years.
The risk is that a multi-volume project can outlast its author, which has happened often enough that publishers plan for it.
New access restarts the whole enterprise
Records open on schedules, private collections are donated, and participants eventually speak, so material appears decades after the presidency ended.
Each release justifies another biography, which is why figures already covered extensively continue to receive substantial new books.
The genre is therefore cumulative rather than settled, with each author writing against a shelf of predecessors and the documents they never saw.