It's simple. The rules for creating magic armor (Craft Arms & Armor feat, DMG p. 179-182, etc.) allow the creator you to pick a type of armor to add enhancements and special abilities to. Robes (a) do not apear in the PHB "armor" table, (b) do not appear in the available armor tables on DMG p. 180, and (c) take up a completely distinct slot type from armor on DMG p. 176. (As opposed, to say, a buckler which does appear listed on those tables.) Robes are just not armor, and that's all there is to it -- the fact that magic vestment makes a special exception has nothing to do with permanent magic items.
As usual, there are obvious major advantages if the DM allows armor enhancements to be thrown willy-nilly on anything a player desires. For starters, wizards suddenly get to appear unarmored while their robes give them the equivalent of free glamered armor (normally a +1 bonus special ability).