I duo bot with my friend in ranked, we play exceptionally usually with 10 kills to 4 deaths between their jungle, support and carry. the rest of the team does the opposite. Leaving lanes with a 10-x carry and seeing mid and top 0-5 each or so every game makes it tough. We are skilled enough to carry some games and win.
The other reason is that 50% of my games have a DC for my team, half the time it is a 4v5 and it is fairly impossible to win every 4v5 game by itself but them having 2 of the 4 contruibute to most of the kills makes it certain defeat. once Riot finds a way to judge individual skill I will be interested