I believe luck plays a role to an extent, but overall a lot more games are winnable than you'd think.
League is a very snowbally game. I used to always get the mindset when another lane gave up a couple early kills that I had to start getting kills in lane and hard carry. I'd overcommit, get caught by their jungler and die myself. We'd fall farther and farther behind and eventually lose - nearly every time that either one lane gave up 3 early kills or both other lanes gave up early kills I'd lose.
I started really not caring so much how other lanes did. I played my game the same regardless. I noticed I started making a lot more comebacks. I started having a ridiculous conversion rate on those games.
I think what happens is players have different skills. Some are really strong in the laning phase. Some are stronger mid game or late game. Some have better objective control, better game sense, but worse mechanics. It isn't uncommon to get a couple people on your team that are weak laners against a couple people on the enemy team that are strong laners. They're going to fall behind. The key is to give them enough support to stop your team from falling too far behind, because come late game those strong laners make worse choices and your weak laners are better players. They'll win from behind. Too often though, you feel like your team just sucks rather than recognizing a potential strength, and you start losing too. At least that was my experience.