I think you are missing the point!
The 50:50 "ideal" in eSports is about the "playingfield". For example your soccer example
Every team plays on the same field with the same ball and the same goal. Everything is the same. Yes, some teams will always win. But thats because the players in the teams are different! And those are the variables, that can change and make a difference!
If soccer was not like described, but the field was tilted by 5 degree in one direction and the goal "at the top" is 2/3 as wide as the other one and also we have only one hour of play without switching sides ... would that be fair to the team playing "uphill"? No! The game would not be decided by the better team, but most of the time, who plays on the "better" side!
In online multiplayergames it is sometimes very difficult to even out those playingfields!
Examples:
In SC:BW, because there were no changes to the races anymore, the map-designers actually were important to create maps, that are "even". Many maps are called "favored" for a race, because they have a higher win rate and a map, that is 50:50(!) over all matchups is hailed as great and people want to play it! Just playing on the same map was not enough, to balance the races!
In CS
GO) not only the two teams have different starting points on the map, but also some weapons are only available to one side. I'm not that into the game atm; but quite possible the "discussion" if the AK (terror side) or the M4 (ct side) is the better weapon is still ongoing. But the teams have to switch sides, so any "problems" in that regard are quite minor. It comes down to execution and strategy.
DotA/LoL/HoN/... has many different heroes with different skills and strengths and weaknesses. Yes, there is a draft in the beginning and a team can "outdraft" an opponent, but it would be really bad, if one team gets a hero and everyone watching would be "oh, ok, they won now!" just because it is so strong in every regard. If so and teams with that hero would have a 75% win rate... you can't put that on skill difference anymore! People will be bored!
The last sentence is maybe also a very important for Online Multiplayer Games. If the game gets boring, people don't play anymore. In DotA there was a time, the first 8 picks/bans would be in the area of 10 heroes. They were the strongest. And people were like "oh yeah, this again..." and I don't know about 50:50 winrates during such times, but the game was changed, just to "spice" things up! And that is the difficult part!
The 50:50 "ideal" in eSports is about the "playingfield". For example your soccer example
If soccer was not like described, but the field was tilted by 5 degree in one direction and the goal "at the top" is 2/3 as wide as the other one and also we have only one hour of play without switching sides ... would that be fair to the team playing "uphill"? No! The game would not be decided by the better team, but most of the time, who plays on the "better" side!
In online multiplayergames it is sometimes very difficult to even out those playingfields!
Examples:
In SC:BW, because there were no changes to the races anymore, the map-designers actually were important to create maps, that are "even". Many maps are called "favored" for a race, because they have a higher win rate and a map, that is 50:50(!) over all matchups is hailed as great and people want to play it! Just playing on the same map was not enough, to balance the races!
In CS
DotA/LoL/HoN/... has many different heroes with different skills and strengths and weaknesses. Yes, there is a draft in the beginning and a team can "outdraft" an opponent, but it would be really bad, if one team gets a hero and everyone watching would be "oh, ok, they won now!" just because it is so strong in every regard. If so and teams with that hero would have a 75% win rate... you can't put that on skill difference anymore! People will be bored!
The last sentence is maybe also a very important for Online Multiplayer Games. If the game gets boring, people don't play anymore. In DotA there was a time, the first 8 picks/bans would be in the area of 10 heroes. They were the strongest. And people were like "oh yeah, this again..." and I don't know about 50:50 winrates during such times, but the game was changed, just to "spice" things up! And that is the difficult part!