Well, first of all, aside from Zhonyas, there actually ISN'T any armor itemization available for mages (or ADCs, for that matter). Magic pen boots are generally too good to pass up (although against multiple ADCs or an AD assassin mid I do favor Tabis).
Anyway, in a straight-up 1v1 duel, mages tend to have an advantage over ADCs; with a few exceptions (Karthus, Azir, Fiddlesticks) most mages' damage is heavily front-loaded. That is to say, they choose a target, blow all of their cooldowns (dealing all of their damage very fast) and then have to wait until their abilities are back up to contribute again. ADCs, on the other hand, deal much higher damage over the course of a teamfight, but generally need longer to do it.
What this means is that survivability is much more important for ADCs than it is for mages. Say itemizing defensively buys you an extra five or six seconds in a teamfight. For an ADC, that means a bunch of extra autoattacks, potentially adding up to thousands of damage. For a mage, though (again, with some notable exceptions) that's perhaps one or two cooldowns back; since your damage is cooldown-based, living longer is often not as important as making the most of each use of your abilities.
tl;dr
Mages tend to fire off all of their spells at once, so their primary focus is making that initial blitz of spells count as much as possible; living longer doesn't mean much if your cooldowns don't come back up in that time. On the other hand, ADCs deal damage consistently over a longer time, so building defenses greatly improves how much they contribute to a fight. Basically, mages don't build as many defensive items because they can't really use the extra time well.