I honestly do not mean this as snark but no matter how hard I try to explain it to you, I cannot understand it for you.
You are correct that you cannot make me share your implicit assumptions by merely repeating your conclusions at me.
I don't know if it's because you haven't played a lot of competitive sports (and I don't know if you have or have not) or if you simply have difficulty with empathy or imagining how other people feel in certain situations unless those situations are familiar to you. Again, I don't mean this as snark or criticism. It's an observation, and it might be more reflective of my inability to explain something to you rather than your inability to understand or your determination to cling to your position without actually considering the validity of someone else's points. I honestly don't know. But I'm giving up.
It beggars belief that you would speak to me of empathy and considering another viewpoint. You assumed the coach was an 'asshole' without entertaining any other possibility.
I played board games with my nieces and nephews over Christmas. I sometimes played not as strategically as I could have, because I have decades on them and it isn't fun to lose all the time, especially when you are in primary school. I am an adult and they are children. But if they were playing against peers, why on earth should they each not play to their utmost? That's what I thought children were taught about sports: it doesn't matter much whether you win or lose, as long as you try your hardest.
I made no assumptions about the coach. Allowing your team to run up a score against a dramatically lower skilled team by such a wide margin is simply an asshole thing to do. It is. This is widely accepted among decent people in the US. It's poor sportsmanship.
You understand this on one level: You play at a lower level when you play board games against your nieces and nephews. You have empathy with them: you know it feels lousy to lose all the time. That's part of the issue.
The other part is: what do you teach your nieces and nephews if you play at your highest level and consistently beat them--badly? You teach them that they are no good at whatever game it is that you're playing --and more importantly, perhaps no good at anything (kids over generalize especially when they feel criticized by someone they admire and respect, like an uncle) and that you're not very nice.
My husband's grandfather was a pretty good chess player. When my husband was a child, his grandfather taught him to be a pretty decent chess player---by playing with fewer pieces until his grandson could hold his own, at least a bit, gradually increasing his own level of playing as his grandson could withstand a bit--and taught him not only how to play chess well, but sportsmanship and compassion and encouraged him enough not to give up but to keep trying, that he could learn to do better.
It's not different in competitive sports. If coaches taught skills by punishingly beating their students by outplaying them at (insert whatever sport you like), the kids would likely give up before they actually learned. Instead, adults--parents, coaches, teachers, uncles, etc. teach children (or lesser skilled adults who may be their peers in other respects or even superior in some skills) teach a little at a time, meeting the student at the student's skill level and then helping them gain more skills.
Competing against other teams is an extension of these lessons--it's applying these lessons. Among those lessons should always be, at the very core, sportsmanship: not drumming your opponent because you can. The very good players learn nothing good by running up scores. They do not improve their own skills. They do not improve the skills of their opponents.
A good coach will substitute in less skilled players who normally don't get much play time, both to give them a chance to improve their skills and also to be more fair to the opposing team. There are other strategies that I've mentioned in other posts that good coaches will do: insist on a certain number of passes before a shot, insist that shots only be taken from such and such a position on the court and so on.
Look at it another way: Suppose I wanted to play in a tennis match and suppose I knew how (I don't) and was reasonably good for my age. Suppose I entered the court and found I was facing Serena Williams. There is no way that I would not be seriously drummed even if she played with only her off hand and blindfolded. Unless she adjusted her play to give me a chance to get some kind of volley going and actually learn something I didn't already know: Serena Williams plays much better tennis than I ever could, even if I had started playing when I was 4 years old. I might learn some skills and Serena Williams would not look like a grandstanding egomaniac taking advantage of a very amateur player.
It's honestly the same thing as you toning down your play to give your nieces and nephews a chance to win. Only more so. You are doing it not to discourage your nieces and nephews and so they can have fun. A good coach will do so for the same reasons and also to give his own usually benched players a chance to play and the other team a chance to play against players more evenly matched--which will build their skills, too. If you are badly outclassed, you don't improve your game. Usually, players give up and sometimes lose their tempers. Because it doesn't seem fair. Kids of all ages, and adults as well, respond much better if they think things are as fair as possible.
Nobody is suggesting that a team should throw a game to make the other team feel better. I think everybody understands that players want to win their matches.