Because the opposing Top laner is 13/4/8 while you're 8/5/3. Yes some of that will absolutely be due to the fact you lost, it's to be expected your lane opponent will have a good score just because he was around while your team got wrecked, but if you're that strong than don't be afraid to group up with them. A whither on an ADC(especially a super auto attack relient one like twitch) will basically shut down their damage for a team fight.
Now lets be completely honest: there will be some games you just can't win regardless of what you do. Absolutely true, and maybe this was one of them. The secret to climbing is not winning every game, carrying every game, it's winning more than you lose. Learn to shrug off the last game, learn when you make mistakes, and next game try to remember what you learned. I've had games where my team gives up 2 kills a minute for the first 10 minutes, I'm not going to expect to win those games, but I know that I'm also going to get games where I'm on the other side of the stomp. It's the close games where if you're better than your currrent rank, you get to be the difference between winning and losing. Remember that for every top laner you stomp because you're better than them, there will be a mid laner on your team that gets stomped because their opponent is better than them. You're not the only one that's working to climb, or get to their "correct ELO", you're going to be up against last season diamonds that had 8 placement matches with leavers, and plats duo'ing with their silver friend, and just because your lane opponent is a gold or silver because they deserve to be there, doesn't mean your adc isn't against a diamond's smurf or a platinum player that only plays 10 or 20 ranked games a season.