For me yes, the constraint is losing the bookmark links encoded in the PDF. As the sources of these books are unknown (or questionable) I don't have any information on the motivation of the creator to work in such a manner. I suspect drugs
Seriously though, they must have had some constraint related to file size (some are over 100mb) and performance or just the body of work was so large it was completed over a lengthy period and therefore in sections, maybe even by more than one individual. Who knows?