How come AD assassins get hexdrinker while AP mids get jack shit?

You're telling me to rush some 1200 gold item that barely helps with the AD burst while solving absolutely none of my mana problems? Yeh that 40 armor is gonna help with the Zed who never runs oom and keeps chunking me with shurikens all laning phase. That 40 armor is really gonna put a dent in that Talon's instantaneous 600 damage combo. Meanwhile the AD assassins rush hexdrinker and become unburstable every 90 seconds, while AP mids are afraid of using 2 spells to retaliate in fear of running oom.