Ad champs get because all of the following can be proc with auto attacks.
-press the attack -lethal tempo -fleet foot -dark harvest -electrocute -predator -phase rush -summon aerie
While mages only get -arcane comet -phase run -summon aerie -electrocute
While mages can activate lethal tempo, the damage part is only through auto attacks.
buPPDrmXEO3 votesAnd Bruisers have 0
Kaìju2 votesHave you tried kite mages with Footwork? It's niche sure but I had a lot of fun and really liked Footwork Sol and Ahri because it ensures you a lot of health. On average I get more health back taking Footwork on an AP than an AD champion. Plus precision gives access to the mana rune so kills and assists make you pop off with semi infinte mana that you can chain to other kill participations.
Pokemonred2001 votesDoesn't Fleet have a 40% AP ratio? Since AP can get much, MUCH higher than AD can, I can see it being useful on a few mages that can basic attack occasionally (or frequently, in the case of Azir), especially if they can use the other runes (Tenacity is useful for everyone, I'm pretty sure Azir would like the attack speed, Presence of Mind's mana regen is actually pretty dope, and Coup de Grace is honestly ridiculous for burst).
I gotta try this on 
and 
now, lol.
Infernape1 votesThe problem is going Fleet Footwork on mages gives you attack speed (due to the precision tree's bonus). No mage would find that useful apart from Diana, Azir, Kennen, Lulu and Twisted Fate.
Although it does benefit AD champions (especially ADCs) slightly more than mages because they'd all have use for more attack speed and because Fleet Footwork can crit.
Kaìju1 votesYou forget though that a lot of mages have really nice auto attacks. Vel'Koz has a very smooth auto, as does Sol, and other mages too, especially some like Annie or Zyra, Zyra I know having 575 range and Annie might
Pokemonred2002 votesDark harvest is honestly kind of fun on AP champions with empowered AAs like Viktor, Zoe, and Evelynn, since if you're building AP it actually does magic damage and you get a soul from every kill you get in a teamfight or cannon minion that dies near you. I don't see Predator on too many champions but I will say it's a pretty nice rune for AP and AD alike, although Phase Rush I feel is honestly kind of crappy as far as runes go.
Dead flag blues1 votesI enjoy picking Predator on Evelynn (even tho I don't play SR a lot), Lethal tempo is ok on Azir, Electrocute on AP assassins / burst mage (Akali / Ahri / Leblanc / Kata / Ekko / ...), Phase rush is trash even on Ryze I agree, and Dark harvest is great on Gragas and Evelynn too
Eedat1 votesI guess it depends on what you define as "mage" and "AD". It's easy to make a comparison when you are comparing one specific subclass to a ton of subclasses clumped together as "AD". For instance, no Marksmen take phase rush, Electrocute, dark harvest or predator. Similarly no divers take fleet footwork or lethal tempo. But if you lump the two plus some other classes together you can make a list that seems unsure when in reality most classes have a more or less equal number of usable keystones.
Deathbypie1 votesWhy can't ad champions take arcane comet too?
vgamedude1 votesMaking FF scale off of critical on top is a mistake imo. Not sure why the super carry auto attack adcs have such cheap and easy and abundant sustain.
