Mage items have terrible built paths.

Mages items has such terrible built paths compared to most AD items. A mage needs to fully complete a void staff to get some magic penetration for 2650g, while AD can get a smaller version with armor penetration for just 1300 gold. A mage needs to completely finish zhonya's to get the lifesaving passive for 2900g, while AD champions can get a quicksilver sash for just 1300 gold (ok a mage could buy this one too). A mage needs to finish their luden's echo for 3200 gold to get extra burst damage, while an AD champ can buy a Kircheis Shard for only 750g.

Sure most mage passives are a lot stronger than the ADC ones, but why does most items have no smaller version of its passive included in their built path? The difference between having all the parts but not able to complete an item, and completing an item is massive. This terrible built path together with the massive dependency on completed items and their passives (like 35% extra AP from deathcap, Zhonya's active, Void staffs magic penetration), makes a mage that falls behind, especially if it mainly brings raw damage to the table, feel useless.

So is there a reason why there isn't a mini a void staf with less magic penetration, a mini deathcap which gives some extra ap, a mini Rylai's Crystal Scepter that slows for less or a mini zhonya's (a mini zhonya might not be the best idea, but ok), while most other items have, especially AD ones?