Logically, a mage is not going to be on the front line. It would be a waste to produce armor catering to them, as would it be a waste for them to seek to acquire it. The school of magicks they know are more often than not obtained via studying. And with the countless hours they have poured into these tombs, they missed the opportunity to craft their bodies into some Greek statue.
Lighten armor? Sure. But they're only magic. Magic is a reality in their universe. If they lighten the armor so it is easier for them to wear, then it's also going to be less effective. The sort of reality bending paradoxes in fantasy universe, the sort of force capable of creating impenetrable armor that is light as a feather, that's something left to the "gods". A mage simply doesn't reach that level. And when they are at that level, they're probably wielding magic capable of leveling a small continent every time they blink, so they don't really need the armor.
And sure, it's not that hard to wear light armor. But just because the mage is going to be fine in light armor doesn't make that armor more effective against the legions of musclebound adversaries. If the mage isn't going to be properly equipped for mass physical onslaught he's about to face, he shouldn't be bothering with armor at all.
So, why robes? Because the robes are typically the uniform of scholars. Not mages specifically, but any of the studious sort in a fantasy setting. The ivory towers are filled with these men in robes, and the mage is no different than them. Just because the mage's pursuit of knowledge has given him powers doesn't mean he no longer identifies as a pursuer of education. That, and it's easier to enchant cloth than metal because it's less dense.
It's hard to do, but you have to apply "reality" to the magicks. Not our reality, but the game/setting's reality. Yes, the mage has access to numerous spells and powers. But he's not a god and their are higher forces. Random revivals and constantly sewing up wounds isn't as easy as just "curaga!!". If you can accept mage's having those dumb powers, then you can accept that they just don't wear armor because they like dresses. Otherwise, step back and think about it logically. Wearing armor is simply not something the typical wizard needs to do. And if he does, that's why there's battlemages.
