If the enemy team has a Pyke, Zac, and Cho, don't expect your ADC to be able to dodge them all.
If the enemy team has a Kayne and an Ekko, don't be surprised when your ADC gets popped.
If an enemy Tahm Kench flashes on top of your ADC and he is allowed to proc his passive damage, don't even bother pinging when your ADC becomes fish food.
In a Meta where complaints of high damage and low cooldowns the class with the least mobility and utility still catches the most flack for getting dove on.
Even Ezreal with his ability to rush an Iceborn and dash away with his E is not immune to assassins.
In the current meta assassins are cutting through tanks like butter and tanks are designed to counter them specifically.
Meanwhile, assassins are built around the idea of blowing up the ADC.
Not to mention bruisers, divers, and other class ranges that can effortlessly blow up an ADC if they managed to get to them unless the ADC is unimaginably fed.
So if your team doesn't lock in a single CC heavy champ in the entire lineup, expect your ADC to die.
If the only dive stopping CC in your line-up is a Pantheon stun and he's using that to engage, expect your ADC to die.
If you've locked in Karthus support and are using your slow-wall to engage rather than to peel the Pyke off your ADC, expect your ADC to die.
If the only CC on your team is being used to engage, expect your ADC to die.
The best marksmen in the entire world, sitting on the worlds and MSI stages, cannot survive fights without CC and frontline to enable them. Every composition at the pro level has CC of some form because it makes or breaks fights.
Positioning can only get you so far; if the best players in the world can't get by on just positioning alone, how can you expect your ADC to?
I don't play much ADC. I'm a fill main so primarily I play Support, Jungle, and Top; Depending on the current meta and which roles people just don't want to play. However, when I do, it seems I'm guaranteed to be stuck with a lineup without a single ounce of peel. I keep quiet and I keep calm. As a fill player I don't mind playing at a disadvantage as long as my team is ahead. My team though, always seems confused when we don't have the CC to keep a Diana or Katarina off me that I die. Even the safest ADC's do not match the mobility on their own, they need peel or damage from their team to burst the assassin or diver down. The team engages hard in the enemy ADC, I'm positioned towards the back-middle of my team as I should be (since it offers the most protection and access to the enemy team, specifics situational of course), when the enemy Zed slips past the team and ults me. Not a single member of my team turns, I dash, flash, and heal away but in many cases the raw base damage from the ultimate is enough. Even if it's not, the Zed has another hop and his own flash remaining to finish me off. This does not necessarily bother me. If I get taken out, but my team wins the 4v4, we are golden.
But then the MIA pings start to fall. "Trash." "Garbage" "Silver" "Bronze" "Are you for real?" "Did you buy your account?" The team manages to lose the 4v4 and they blame the fact that I am not doing damage. Me. The ADC they didn't even make an effort to help surely should be able to 1v1 a Zed or a Kayn on his own right? No. That's not how this game has ever worked. In no elo, during no patch, in any season has an ADC who is not absurdly fed been able to 1v1 an assassin or diver unless the assassin/diver misplays the fight so horrendously they were earning their way to a demotion to begin with. You need CC to pop these people and you need the team to focus.
When I play support I often pick up Lulu, Janna, Zilean, or Taric for the very purpose of denying the enemy diver the kill. If Zed can't pop my ADC, we win the fights. The difference just one peel focused, anti-assassin oriented pick makes is enormous. Even the likes of Shen and Thresh can get the job done, allowing the ADC to get off their DPS and making fights competitive.
**TL;DR? If you don't stop assassins, don't be shocked when assassins do their job.**
Thank you for coming to my TED Talk.