Heroes 6 - Discussion

I think mages can live anywhere. At least because they have already been moved from snow to desert (and this change is much more significant than from deserts to jungles). So the absence of deserts is unlikely to be a reason for the absence of mages.

The mages of Enroth lived in snowy lands, yes.
But for the mages of Ashan, the southern deserts are their home. It is practically clear that the jungles are the lands of the Nagas. There are a number of screenshots from Heroes 6 that show this (Sveltana's mission with Mother Namtaru takes place there).