The point was agreement. Any other AP item works on maybe one or two champions or is built in niche situations... and the issue is that rounding out your build is difficult because unlike ADC, we don't have a "cookie-cutter" build that works on most the roster and is natural.

Think you can go with those items on nearly every ADC and do all right. Replace Runaan with Statikk or RFC and you fit most of the rest of the ADCs. There are only a few exceptions. (Could probably throw Mercurial somewhere in there too.)
AP Mage itemisation somehow feels in a worse spot where we simultaneously have more and less choice. More, because we could trap ourselves by buying
when we're playing
, and also less because it feels like Vel would only ever pick up Luden's and has no use for any of the other three situational items and doesn't want Protobelt.
Unlike an ADC, we don't get to pick up sustain/defensive AND get top-tier AD (looking at you, Bloodthirster), and we don't get to acquire items like Runaan/RFC/Shiv which augment the way we play while still offering damage augmentation.
When I play mage, I'd rather have a solved setup like ADCs do where we swap out some of the core items depending on who we play and how the game plays out than the current situation, where it feels like you pick a mage and then build the exact same items every game because there's no "I want to situationally grab x" overmuch.
Yes, there's some nuances, but way too little, way too few.