Fine Transmutation Stones Suggestion
It’s like hot dogs and hot dog buns.
We may never know the answer.
They will never do it in a pack of 6 for the very reason you listed OP: 6 = a full armor set. By setting it at 5, the individual then feels compelled to buy an extra set, which after transmuting a full 6 piece set will leave that said individual now with 4 pieces possible, which would require another pack of fine stones to continue transmuting another full set.
It’s a never ending cycle of purchases.
If you do your skins for armor and weapons, you use 10 if you use something like Greatsword/Short Bow, 12 if using Sword/Shield and Scepter/Focus. You’re gonna end up with something off either way.
I’m sure someone in monetization is feverishly working on a way to remove the ability to get 10.