They are balanced around team fighting and reliance on front liners/supports.
Because they stand so far back they are usually isolated from their team to a certain extent, so they are susceptible to flanking.
The second part is that they typically want to team fight, so if you're front line is stronger than theirs you can win without ever having to actually fight the ADC themselves. Playing an auto attack ADC with a weak front line is a nightmare. So rather than thinking of how can I kill that ADC they are so fed, just focus on winning the team fight. If you kill the other 3-4 members of their team really fast they will have a hard time doing anything.
As solo q players we focus a lot on individual champions. "How do I kill that champion?" "Who beats that champion?" "How can I put out more damage than that champion?" But the game isn't balanced that way. So yeah if you want to beat it you have like 3 choices:
- Take an assassin and flank the crap out of them
- Play a full poke comp and attempt to win the game through mid game snowball without actually fighting but just poking them off of objectives and getting picks.
- Play a team fight comp yourself with a crit building ADC and just team fight better than they do (as a ADC/Mid main this is my personal favorite)
Again, you don't have to kill the Tristana/Twitch/Jinx. You just have to win the game.