This is because of the same reason you are not good at anything else you are not good at - it requires practice.

Single player games and multi player games do not in any way translate into the same kind of skill. Testing yourself against the game can be taken at your leisure - that is to say, no matter how many times you fail, you can try again in your own way and at your own pace until you succeed. This, in turn, does not engender any level of efficiency or skill whatsoever, besides very basic things like the ability to aim or move.

When your opponent is human, you are FORCED to be better than your opponent, and you must do so on your first try. There are no re-dos, ever - even in another match with the exact same opponents, the match will be different because humans are not designed to act the same way every match in order to allow you personally to improve your skills. Additionally, your opponents are probably also improving as you play against them, making every subsequent match even harder.

Basically, it's a totally different mindset. Not that there is anything wrong with one or the other, but you can play single player games, even ones considered to be extremely difficult, for billions upon billions of hours and you will still be a fledgling when you step into the multi-player ring. The reason you find yourself struggling with multi-player is most likely because the people you are playing against have just as much gaming experience as you do - but with significantly more multi-player experiences.

This difference is important, because as I said, it shifts the way you think about the game and how your brain approaches it. When there is no consequence for failure, as is the inherent state of all single-player experiences (you can just try again), you are likely not thinking about the same things you would be if you were trying to improve your multi-player skills. Getting better at games does not simply require practice - you can practice until the end of time and not actually get any better. It takes focused concentration and active problem solving to be applied in order to actually improve. You can't just gain experience - you need to think and reflect on that experience and actively attempt to change your game behavior in order to improve. It's a skill that must be learned, it's not just muscle memory, and just like a student who isn't paying attention if you are not really trying to learn something then you won't. You plateau because you have hit the limit at which muscle memory and physical skill will help you, over making good decisions.

A lot of people have shitty attitudes, one of the key things to improving is not actually caring what other people are saying and just enjoy the process of improving yourself. Find methods that work, discard methods that don't - most people who tend to get mad at others for making mistakes, especially if they are new, do not approach the game with the attitude that they themselves can even make mistakes at all and just blame things on others. This means they are bad. Whatever game it is, they are bad. If you can't admit your own mistakes in a video game then you can't actually improve. So don't listen to their advice, because it is bad advice. Just ignore them.

On an unrelated note, you mentioned that some League of Legends characters are better than others - this is not true, and is very rarely ever true of anything in most games. Most strategy games in particular tend to have rock-paper-scissors type balance rather than just making everybody equally powerful in all respects - character A is strong versus character B, and character B is strong versus character C, and character C is strong versus character A. Et cetera. LoL in particular has over 200 characters and it is impossible to really see that - yes, they are fairly well balanced and each character is useful for something or another - without thousands of hours experience, watching and experiencing the hundreds of thousands of possible match-ups for reference. I think it's a great game, but yes - it does take a lot of concentrated effort to play and enjoy. I would say this is true of all competitive games. However, if this is not a style of play you enjoy (thinking about every facet of every decision, every second) then you might just not enjoy the competitive aspect of games. Pursuit of mastery is essentially what makes human beings have fun in competitive environments, but if you don't care for that much then it might just not be your bag of tea.

Anyway, I hope this helps. I was in the exact same boat as you not too long ago, constantly practicing and wondering why I could never beat any of my friends. I had plateuaed and couldn't figure out why. Ironically enough it was League of Legends that helped me past this mental block - there really is no blaming anyone or anything else but yourself when you just run under a tower and get blasted to death trying to chase an enemy. Eventually being able to admit my mistakes translated into literally everything I did in any game at all - playing a shooter, why did I die? Not because he got the sniper first, but because I wasn't paying attention to the map and where the spawns were, and gave him a clear line to both the sniper and my skull. Basically, blame yourself for literally everything that goes wrong, analyze how you could have stopped it or done better, and you will start improving rapidly.

I have a hammer doll! My gamertag is "Krynin", I don't even care about the dolls, I just want the achievement. Message me, yeah?