As an ADC main I feel just the opposite.
I can win early in every single game. It takes almost no effort from the enemy top laner to completely shut down any lead I've mustered and get his ADC steamrolling though. That 20cs and 1 level lead at 8 minutes becomes meaningless the instant the enemy top lane clicks TP and my top laner doesn't respond in kind. Double kill enemy ADC, bunch of missed experience for me, dragon for the enemy. Suddenly the lane I was bullying and zoning is snowballing.
It takes only slightly more effort from the enemy mid laner to put me behind. All it takes is my mid laner not pinging a missing and I'm extended because I KNOW I can win the 2v3 against the enemy jungler but then surprise there's an assassin on my face. Even if I'm warded, it doesn't do much good to see a LeBlanc or Zed or Fizz running down the river when I'm at the enemy turret. Even if I do get back to my tower, I'm getting dove and killed.
Which brings me to your ultimate problem:
ADCs are supposed to be weak early and strong late game. Solo lane champions are not only stronger champions in the early game, but they have a natural experience lead on top of that. If you're NOT using that advantage to help you ADC / shut down the enemy ADC, you're playing wrong and it's no wonder you're losing. You're playing your role inadequately.
I don't think I've ever lost a game as an ADC when the enemy top/mid/jungler ignore my lane all game. Simultaneously, I don't know if I've ever won a game as ADC when the enemy top/mid/jungler are focusing my lane and my allies aren't responding appropriately. There's very little a level 5 adc and support can do against a Malphite+Fizz gank if they only see it when they reach the dragon pit or the tribush.
You guys all feel this way because when you win despite the enemy ADC being better it's in a stomp where you won hard and early and you think it has nothing to do with the role being weak but because you played so much better. You didn't, you just played well during the time you had the advantage. If you fail to do that, it's only natural that you'll lose more often when the game goes later.