There is some theoretical basis for Voynich exploring the manufacture of medieval documents. Voynich had trained as a chemist, and was an antiquarian bookseller, and acquired collections of books and documents. He would thus have had access to loose sheets of vellum and paper from some of the old volumes which were not in a fit state for resale, which could thus be put together to form a composite volume. He might also have acquired old writing materials as part of his trade. (Craft workshops do keep materials of varying age, some of which have been inherited from other workshops which closed down etc; and it is possible to acquire artists materials of a certain age even now.)
Ghe may well have experimented to see how the old manuscripts were created, investigated natural inks and pigments, and seen how different materials reacted to modern chemicals, or even 'improved' a few pages: more generally, the 'creation' of purported documents using old materials is not unknown.
However why would he have created something as long and as 'strange' as the VM? The main reason could be to have something that was obviously a 'created document' or a puzzlebook (so not a fake or forgery).
When would he have had the time to practice the script and artistry of the VM, the layout and other features, especially as it is written in 'a mostly fair hand' and in an unknown script - which would have taken time to acquire. There would also be drafts and failed versions etc.
Where are the draft versions - and why create only one document, rather than a larger cache, or a text which fitted into a known context?
The VM is written in an unknown and undeciphered language. This script and designing the layout would have taken some considerable time to develop - and then the 'showpiece manuscript' would have to be created.
If other people were involved in the construction of a relatively modern document why did they not leave traces of their activities/accidentally let slip that they had been involved.
Most importantly - the document and the ink have been dated to the late medieval period.