Surprise! There's a REASON Hexdrinker is more efficient than unstacked Armguard but NOT stacked.

1. Assassins have poor waveclear early without burning their only safety tool or lots of mana. All mages have good waveclear and an early Lost Chapter offsets any mana problems from there on out as long as they don't spam like an idiot. AKA, they can actually stack Armguard fairly easily.

2. ALL mages are ranged. ALL AD mids are melee. Mages have both unmissable ranged autoattacks and ranged AoE abilities as well as targeted (for some mages). Mages have CC which brutalizes assassins. Read: Mages have a lot better poke than any AD melee in the game.

3. The only way an assassin is going to do anything to a mage is by using their gapcloser. Which has a fairly long cooldown in early and is usually one of the last maxed abilities for any assassin. AKA, gives up waveclear potential and safety/chance of escape if the engage goes badly. Mages have ranged autoattacks to constantly bully any melee with as well as AoE long range abilities which can also be used point blank which are on a lot lower cooldown because it's usually part of their main rotation.

#4. Assassins are meant to be strong early and midgame. Mages are meant to scale lategame as one of the hypercarry classes.