as someone that plays a Magus in Pathfinder. my default armour is. a Silk ball gown topped by a lovely coating of Mage armour and backed by a generous side dish of dodge and dexterity. throw in a dash of Mirror image and a Caster class character is an evasion tank. and get to wear whatever clothes she wants.
But that is't the point of this thread. the question asked "Why do wizards have to wear robes"
A: So the enemies know who to geek first.
Hmm, interesting choice.
Talking about cliches. When you attack an adventurer party then you might be well advised to also kill all females first in look how often they are the standard cliche for healers.:p
I guess it's up to game makers if they want to serve cliches or grow up and allow a wider range of variety in gear.