At a young age the benefits that come from not having an extremely superior team (which happens purposefully in many locations as all the good rec players are grouped up onto one team with the rest generally being filtered off into the 2nd or 3rd one) completely and utterly humiliate you for 30 minutes by running up the score purposefully.
Not to mention, as I said earlier, if your team is THAT superior you're still going to end up winning with your 2nd string in or you end up slaughter ruling them if there is one in your league. Either way, its still not like the losing team got off scot free or isn't going to learn that sometimes you just get beaten bad and there's nothing you can do about it. They're going to learn that lesson, still having their face introduced to the crap that comes with life. But at the same time they're not getting their nose shoved down into that crap by a foot to the back of the head while everyone is laughing and pointing at them, which is essentially what purposefully running up the score will do.
And at an older age, like high school, purposefully running up the score is bad coaching and bad teaching. Its bad coaching to keep your starters out there when its not needed, not allowing your other guys to get experience and risking injury to your starters needlessly. Its bad teaching as it teaches the players bad habits as they start doing plays and actions that in a normal game you wouldn't do, not to mention inherently tactily teaches show boating as running up the score purposefully on a extremely weaker opponent is essentially just show boating.
If you're in such a position that you're absolutely dominating the other team the lesson in regards to "sometimes life sucks" is going to be extremely apparent regardless of whether if you continue to purposefully run up the score. Trying to say its just teaching the other people a lesson is an attempt to rationalize the reason to be a show boating ass.