In other fantasy games I think the fact that magi do not wear armour comes pretty much from Gandalf in LoTR and has nothing to do with history. There are certainly other fantasy games where mages can where armour .
In those other games their is an attempt to balance the classes and so Wizards get powerful spells but poor hp, poor armour and poor weapons. In AM mages are meant to be more powerful so why worry. OTH if it outrages you and your players feelings you can ban mages wearing armour but I cannot think of a rules justification for it you could say large quantities of Iron intefere with magic then of course why can mages effect people in armour.
Another way would be to include an armour wearing skill (I have no idea of the mechanics) which would make it harder for magi to wear armour or at least reduce their other abilities if they want to wear armour.
Not all magi wear armour in the 4 saga I am involved with only abour 20% of the magi wear armour and several of those who don't are combat magi. It is easy enougth to set ReTe effects to render yourself immune to mundane weapons anyway many young magi can cast such spells. Many of those who do wear armour so so for style reasons and rarely find any use for it as most battles are decided by their magic . only 1 mage in those 4 saga resorts to hand to hand combat as a first choice and that mage is a giant blooded mage who knows a huge range of personal enhancement spells (and she still uses a ReTe ward against mundane weapons)
here https://forum.atlas-games.com/t/character-sheets/2034/1 is a mage about 6 years out of apprentice ship who does not wear armour but who can pretty much wipe out any group of mundanes she encounters and shortly she will have a PeTe spell to destroy any weapon touching her so armour is not necessary