Mages have fewer options in general as well. You have a grand total of about 3(or possibly 4) defensive options in the mid lane, but only one is good against the incredibly popular assassins that are in every other game.
Good against other mages, but doesn't do shit to an AD assassin
Again, good against mages, but doesn't do shit to an AD assassin
Good against AD assassins, but the build path is trash against the assassins; the active helps more than the armor, so seeker's armguard is not a good item against one of these guys.
Would be good if it didn't take so long to stack, and doesn't give you any resistances, so you better hope you have armor runes, or you're still fucked.
Meanwhile, assassins have:
Pretty up there in price with Zhonia's, but counters mids really hard right now, while still giving AD and offensive stats. Not to mention Hexdrinker is a lot more useful to the assassin in this matchup than seeker's armguard is for the mage.
Still has a better item in the build path than seeker's armguard, still gives tons of offensive stats, and lifesteal, as well as a free cleanse for if you're against a Mid with consistent cc, such as Lux or Annie.
That's another thing though; lifesteal. Assassins can lifesteal ridiculous amounts, just like any other AD role. When I see a Zed tower dive me, steal raptors and wind up with 3/4 health when he decides to cs while I wait to spawn (after he was seriously low) I want to puke blood. Not to mention one of the items in the game that gives you the most flat AD also gives you the most lifesteal, and a shield. So, all you need to tower dive someone is Bloothirster. Lifesteal off some minions so you have a full shield, tower dive, and you'll survive a solid 90% of the time unless you're facing lulu, in which case why are you tower diving lulu??? That's what she wants!