I think robes should give like a +5 stealth bonus ( or +5 magic for mage robes) and also hoods could protect your identity maybe??
I don't know about +5 stealth... have you tried sneaking in a robe like that before?
Lets just say there's a reason why, historically, thieves and assassins used average to fitted, but not free flowing garments. Causes excessive movement that attracts attention. Which, haha, isn't the usual goal for someone in that profession.