Do you want to know why an additional armor/AP item hasn't been added?

Because it would completely kill anything AD in solo lanes. Mages and AP bruisers would be unbeatable in both top and mid lane.

As for mid lane, it would eliminate AD mids all together. Mages already outscales everything AD in the mid lane, the only thing that truly outscales mages are ranged ADCs (and even that's beginning to become less true). Mages will almost always offer a mixture of ranged wave clear, burst (and in some cases even sustained damage), utility and longer range than any other class in the game. Yes, many of them are immobile but their immobility is hardly as crippling a weakness as it's made out to be.

This is why AD mid laners, specifically AD assassins, are designed to win lane. Because AD mid laners won't bring much utility to a team, their wave clear often doesn't scale nearly as well and in many cases, are melee champions that can't safely clear waves. This is why you pick them, to compete with mages early before mages outscales them. Zhonyas doesn't offer mana regen, so often you can't rush the entire item, just armguard. So this still leaves AD champions with the opportunity to make plays on the mid laner. Let there be two AP/armor items, and you'd never be able to make plays on them because they'd easily be able to out-tank an AD now because I can promise you, it'd become meta to build both Zhonyas and new AP/armor item vs ADs, maybe even in general just because it'd also be good vs ADCS.

Mages aren't supposed to be tanky and having the option to build two armor items would be pushing them towards that. AP itemization already double dips a bunch into HP and even MR in some cases, so the one armor item is all they need. Being squishy is a designed weakness, so when you catch them out of position with their immobility, you can actually punish them.

But go ahead and tell me I'm wrong and that assassins are OP and mages are bad and have always been bad.