Or maybe its because tabi is over 100% cost efficient as a armor item even without the passive.
Maybe its because tabi is actually overpowered, but that would be too hard for you to grasp, for some reason ppl on this forum are allergic to admiting that tabi is overpowered and they always try to find some bullshit excuse why theres 6+ tabis per game.
"Lethality so strong!! thats why everyone builds tabi!!!"
4 patches later where lethality has been nerfed into shit, everyone and their mother still building tabi
"Mercs too weak!"
Wonder whats the next bullshit is gonna be? And no mercs arent weak they are balanced as a boots item like ludicity, swiftness and zerkers to some extent (the later one is also probably a bit too good in terms of gold efficiency, but AS is a overnerfed stat), tabi is too much, it should give 20 armor or even 15 so its only good against ppl that actually rely on AA and not just everything AD.