There can be many reasons, we are not the game developers, maybe there is technical issues if a full rework is required. Adding more convenient choices is cool.
And yes, developers earning profits, how despicable of them to earn money from us! Let us all report this matter to the local authorities!
Kek, keep telling yourself that, mah boi! If they wanted it, coding of this degree could be done in hours, lmao!
Heck, they probably copied Brave Frontier's "faith system" on a whim but be like "holy moley, we struck oil!" when players felt that their valiants were imperfect and made a huge uproar about the cost to reroll for a chance to redeem their imperfection.
"Hmm... how am I to make money out of this?"
After so much discussions and insightful comments that offered respectable and reasonable reroll cost alternatives, they simply decided to make it a quick cash grab by allowing one to choose their non-valiant's aura while testing the waters for an even bigger cash grab - one meant for valiants.