If they did that, then a complete recalculation of Craftsmanship and Control requirements would be needed. Right now, the minimum requirements for 3* crafting cannot be met without overmelding.
Ala Mhigan, plus FC buffs and Food only gets you to: Craftsmanship 1141, Control 1158 (or minor variations of that)
The stats from Yellow Scrip gear are nearly the same as the non-overmelded Ala Mhigan set.
A Nightsteel Ingot requires Craftsmanship: 1500 Control: 1350, neither non-overmeld approaches even comes close to the requirements.
This would severely impact the casual crafter. The casual crafter just does not have the time available to constantly regather and recraft the base item.
OP, there is nothing wrong with the RNG. It is fine, I have many times burn 30 or more materia on 29%. And, justs as many times, I have hit a 6% meld in 1 or 2 attempts.