Letters and books diverged a long time ago
Journals and monographs can look similar today, but they diverged a long time ago and they are fundamentally different shapes. A scientific article is more closely related to a dollar bill than it is a book.
Q: What else evolved from the letter? Levy has more on this
Yates, JoAnne. https://doi.org/10.1177/002194368201900301.The Journal of Business Communication (1973), vol. 19, no. 3, July 1982, pp. 5–26.
As long as outgoing documents could only be reproduced in books, incoming and outgoing correspondence had to be stored separately. p. 14