[fungalreason]

I find it's partly a logistics issue. Mages aren't all that useful at the beginning without spells, so it's often better to keep them at home and research. Then once you've researched a decent amount, it can be time consuming to get them to the front. Many of the more powerful battle mages are capital-only and map move 1, so depending on how you expand it may be several turns to get them where they are needed.