Well, there's a few reasons: robes do not cause any additional stamina drain when sprinting, robes add almost nothing towards encumbrance, and robes do not hinder movement speed whatsoever. It requires 3 perks into Light Armor to remove the movement and encumbrance penalties of wearing light armor.
There's also another issue of making decent armor. To get something comparable to robes like
you're going to need to dump perks into Enchanting to get Extra Effect. Not that those perks would be a waste or anything, just that there aren't any unique light armors that are designed for the benefit of spellcasters. Even basic robes have strong magicka regeneration bonuses as a baseline feature.