An exception is made for “qi”, just because. Mine is not to question why, simply to regularly lose.
I am currently in Boston for a tournament - the Can-Am (Canada vs USA championship).
Casual Scrabble is all about coming up with fairly well-known words. Serious Scrabble, I presume, is about controlling board position armed with a vast knowledge of legal words.
As soon as someone in a casual game plays a questionable esoteric word that, if challenged, turns out to be a legal word, the casual game becomes a lot less fun for everyone involved.
Presumably they wouldn't devote that much energy to something if they didn't regard it as such.
They also probably aren't high level players in enough other games to give a credible comparison among all the possible contenders, simply due to the time required to become high level in multiple games.
The same is true for a lot of great party games. Scattergories is a great example. As soon as it becomes seriously competitive, the key rule (that everyone votes whether to allow each submission) effectively ruins the fun.
It breaks down somewhere in the middle, where you play with your group of friends but want to be competitive, and it turns out someone gains a huge advantage by learning all the 2- and 3- letter words, all the "Q without U" words, or whatever. One imperfect way to address this is to print out the 2- and 3- letter words and make them available to everyone.