you are really underselling just how good the active of zhonya's is. you are invulnerable and untargetable for 2.5 seconds. that is a long time in a team fight or in general. sure if you are isolate it won't save your ass, but it can definitely stall.
I mean look at champs that can already stall and how much more obnoxious they are with zhonyas just because you simply cannot attack them for like 5 seconds in a team fight.
will E > zhonyas and thats like 3 or 4 seconds you cannot hit the little guy. nevermind he still has protobelt and Q to get out as well after and ya know, flash.
will pool > zhonyas > all while staying untargetable while he lets his ult pop healing him for huge amounts.
can engage with massive aoe damage all while healing and being untargetable/invulnerable and then go untargetable/invulnerable again right after . all while slinging spells inbetween very quickly.
I mean it literally makes you immune to damage for 2.5 seconds. lets say you dash over a wall. have a dot on you? ZING. you are gonna live now. you got caught out but your ally is Tping? ZING. you just stalled for over half the duration of the tp channel.
Zhonya's is VERY fucking good. its so fucking good that the item had to be nerfed into oblivion and become an item bought solely for its active. when it was good it was bought literally every game by every ap champ because the active was strong and the stats were strong.
You also undersell that AP champions have access to high damage and high health items that AD champions do NOT have. if ad champions want health/damage they sacrifice quite a bit of attack damage for it.
only gives 40 ad.
only gives 30 etc.
where as other ad items that are big ticket give 65-80.
ap champs do not make that sacrifice with health and ap items.
= 80 ap
= 100 ap when fully stacked (thats the highest ap value outside of dcap/seraphs you can get!)
= 75 AP
AP champions have the luxury of building whatever the fuck they want based on the situation where ad champs genuinely sacrifice more damage for the defensive actives and passives.
that is why ad champions have more options because every single one has a trade off.
GA is super not cost efficient and honestly terrible with the active down.
Maw doesn't nearly give the same AD as other big ticket items and will gimp your damage at the expense of saving your life from burst.
Death's Dance might be the only item I feel is broken. but the item is only situationally broken. its super strong on anyone who can keep healing out over the bleed. otherwise it just stalls your death, it doesnt actually defelct any damage. BT's overshield is not nearly enough by itself to save you from burst, it just helps a tiny bit. so i wouldn't call it a defensive item. it is a pseudo defensive item for risk/reward style adcs who want to sustain while taking some hits but not getting caught completely.
edge of night just isn't what it was before nerfs. its viable, but 55 ad again is a lot lower for a big ticket item and how much gold you spend into it.
AP champs simply have more options on what to build. where as ad champs have to build pretty much "what they need and maybe a defensive item".