I've seen so many people make the "Smash is equivalent to Boxing" comparison, but that isn't true. Neither is it the same as chess.
What makes a good chess player is 100% mental skill. What makes a good boxer is something along the likes of half physical strength/reflexes, and half mental skill. A good smasher relies on MUCH more mental skill than physical reflexes. If you don't believe that, take for example Borp, a player with literally no tech skill whatsoever who still wins tournaments via excellent spacing, reads, and adaption.
This means that when you are coached in smash as compared to boxing, it can have a much larger impact on the outcome of a match.
Additionally, a boxer can only see his opponent when he fights. He has a limited view of the fight compared to an outside observer who can see both fighters. When a smasher fights, he sees exactly what any outside observer sees, and a good smasher who has the skill to adapt should be able to come to the same conclusions about their game as any outside observer of the game. If they can't, that is a weak point of their own game.
Lastly, unlike boxing, coaching is not regulated in the smash community. Unlike boxing, not every single person in a tournament has a coach, and it is unreasonable for that to be expected of people. How fair is it to have two people fight, one with a coach and one without?
For example, you are good at the game and you bring your friend with very little experience to a tournament. He is in pools and is playing other people with little experience. Between sets, you tell him what the other guy is doing wrong, what moves he should spam, that he should shield grab the opponent more because he always short hops in with an aerial . This simply isn't fair to the other guy who doesn't have a coach because he is being expected to come to these conclusions by himself.
And for those of you arguing that it makes more more intense sets/matches, this is a competitors sport, it doesn't necessarily exist to make spectators happy.
In conclusion, coaching would only be "fair" if there was some way to ensure every player had a dedicated coach for every game, which is unreasonable, and would still take away from the game rather than add to it by removing the mental aspect of adaption from play.