because it's SIMPLY a race to see who can finish the same items first.
All lanes are a race to finish the same general items, and ADC do have a fair amount of diversity - you can rush AS items like Hurricane on Kalista, Kingblade on Kog/Vayne, Brutalizer/Ghostblade on Vayne/Lucian, Tri-Force on Corki/Ez, Bloodthirster/IE on Graves/Cait.
It's not the same item 100% of the time, ADC itemization has vastly improved since season 1, and you've pretty much always got 2 evenly powered options at every phase in the game for their choices.
ADC 1 got a kill before ADC 2 with approximately the same CS? ADC 1 wins, because they can get a BF sword right away. the support only matters for the first kill and to place wards to prevent ganks.
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Supports are among the strongest early-game champions in the game, ADCs among the weakest.
Supports aren't helpless and weak and played as supports because they're good at placing wards, supports are supports because they're the single strongest champions in the game without relying on item scaling, which tends to translate into extremely capable powerhouses of a champion early when all you have to work with are bases. Most supports are incredibly powerful when they do build a lot of items as well.
On this note - you can easily force a weak-sauce ADC to win their lane if you're playing a support, where you're pretty much screwed if you're an ADC and your support has no idea what they're doing.
there's no diversity in the bot lane champion pool OR the item paths. edge, shiv, bloodthirster, youmuu's, last whisper, maybe a GA. it's boring, it's dull, it's stagnant, it's repetitive, and it's absolutely ridiculous
There's pretty much as many ADC itemization choices as there are for any other class at the moment.
Ditto for champions.
Just because meta-sheep stick to the gold, silver, and bronze picks doesn't mean you have to. People do that because they're trying to emulate the single best strategy, not the 4th best - but if you're blindly fallowing pros and hoping for a get-rich-quick strat, you'll usually miss all the important details that such a pick relies on in the first place. I always find it silly when the meta grabs hold of lane-swap, fast-push picks in yolo-queue. Or when they pick early-game, 20 minute win picks and proceed to sit on their butt and farm for 30 minutes.