Since when? Sorcerers started as a different-mechanic wizard in 3e. 4e made them the heavy-damage arcanists, but still very much ranged ones.
If you give wizards Mage Armor as an "Int instead of Dex" class feature, you can add the same for sorcerers and warlocks... if they lack other means of protection. If Hexblades begin with, say, Medium Armor Proficiency, then they don't need such an ability. Sorcerer armor proficiencies might vary by tradition.
And clerics that follow deities that don't give out armor proficiency (like the Arcanist, and IMHO, the Lifegiver) should be able to add Wis to AC (call it Divine Grace or something).
Ok, in the original 3e phb, there were very little things that would send sorcerers into melee, however those traces were there:
-Starting equipment. Sorcerers were suggested to start with a spear.
-Proficiencies, spears, maces, sickles, morningstars, A sorcerer had incentives to go into melee and show off the variety of weapons he could use to the wizard
3.5. PHB: The original suggested sorcerer bonus feat was replaced with Combat casting
Unearthed arcana: Battle sorcerer variant, no melee wizard variant
Complete mage: Stalwart Sorcerer variant, no melee wizard variant.
4e. Sorcerers implements are melee weapons too.
Sorcerers get 1 melee at-will, Wizards get none
Dragon sorcerers are in fact gishes, with a strong melee basic attack and with strength to AC
All Dagger channelers are gishes too.
Sorcerers get lots of close burst and blast (as in be in the middle of th action) powers
[notranslate]Pathfinder[/notranslate]
A number of bloodlines grant melee attacks as bloodline powers.
As for making mage armor a class feature that grants Int to AC, cha for sorcerers and whatever for warlocks, no that would represent a very gamist and enforced needless simmetry, and that is without taking into account how it complicates multiclassing, or even natural character progression (does a white mage that gains armor proficiency lose the wis bonus to AC?) so no, just no.