Because the way flushes work has nothing to do with how many cards are in it, but rather what the highest card is. That's why it can be modified to fit variations with smaller hands, like 3 card poker. Straights are same deal, highest card wins.
And as I said, I have been in a casino that already does 3 pair hands. It just isn't widespread, and that's my initial question. Why not? So far nobody has a reasonable suggestion as to why.
Yes they have. You're limited to 5 cards. If you want to do poker with 6-card hands, invent a new game. Call it 6-card-hand poker or something. The odds are different, the potential hands are different, the highest hand would be a 9-10-J-Q-K-A of the same suit, etc. Poker may involve any number of cards but the one thing all poker games have in common is they all use the same 5-card hands. You might have shared/community cards, extra cards, extra hands, but they are always 5-card hands.
There is no difference between three pairs and, say, two triplets. Or an "extra full house" of quads + a pair.
You are the one who wants to add an extra allowed hand to the pool of possible hands. Justify why three pair gets precedent over a 6-card straight or flush, or two triplets, or 4 + 2.