Mattachine
Adventurer
Of course, if armor weight/encumbrance/movement and carrying capacity were done a lot better, this wouldn't be much of an issue. Few wizards would have the strength/con to wear armor all day.
My first thought is that, from the world building perspective, you can say that wizards simply don't have time to learn to use armour because they're too busing being scholars of magic. (Mechanical translation: they don't have the build resources - whatever excatly those might be - to get armour proficiency).
Is there a reason that wouldn't work that I'm missing? The question is genuine, because in my 4e game the wizard and sorcerer didn't wear armour precisely because they had better ways to spend their feats. So the mechanical story and the ingame story mirrored one another in the way I've just suggested, and at the moment I'm not seeing why my experience wouldn't generalise.
But to make that happen, there has to be some disadvantage to wearing armor or an advantage to not doing so. The former is traditional, and tradition is important.
If you're going to go adventuring where combat is likely, you learn to wear armor.
No, the only reason a wizard wouldn't wear armor in combat is if he or she can't.
... unless you've got more important things to learn and therefore do not spend your feats / skill points / build points / whatever you've got on armor proficiency.
Or, maybe, you learn to wear light armor effectively. Congrats, you now have the mundane equivalent of the Mage Armor spell / Bracers of Armor that you were going to have anyway (except, not being a [force] effect, it's not as powerful; saves a 1st-level spell slot each day, though, so ... woo?).
In short, why doesn't your argument apply equally well to weapons? If you're an adventuring wizard, you should learn to defend yourself, and therefore you should spend a couple minutes with the fighter each night before bed and learn how to use all martial weapons. So, the wizard class should have every weapon proficiency, too.
Most wizards can't wear armor in combat because they aren't strong enough (strength/constitution combination) to wear it all day, and won't have the time to don armor right before a combat.
Most wizards can't wear armor in combat because they aren't strong enough (strength/constitution combination) to wear it all day, and won't have the time to don armor right before a combat.
There's nothing that forbid a wizard to put a Plate, take a two handed sword, and go to fight all the day.
Not every weapon, but a handful,
Even so, weapon mastery is a far more significant skill than learning to wear armor.