The Voynich Manuscript is a cipher because...?

[nickpelling]

I'll agree that a complex cypher is improbable around the time the VMS was made. The ones I've seen from that time period are pretty straightforward.

The notion that Voynichese necessarily uses a complex cipher is just a presumption - until we decrypt it, we won't know if it is complex or just cunning (i.e. several steps sideways from what we are expecting). :-)