Voynichese is a numeric cipher?

[Jorge_Stolfi]

Why are so many people so convinced that the Voynich Manuscript is a cipher?

The reason seems to be that, if it was merely a plain text in a funky alphabet, (1) it would have been cracked 100 years ago, and (2) its alphabet-independent statistics (like the word length histogram) would be similar to those of a natural languages. And the language is assumed to be "European" on account of the material, layout, and illustrations.

By that reasoning, it must be either encrypted with some non-trivial method, or it is nonsense.

How many known 15th century cipher documents are there that are full of illustrations?

None, AFAIK.

All the best, --stolfi