It was precisely this issue that prevented the Andrato Kingdom from grooming Masters easily. One had to understand that the gulf between them and Darius was deep, and Caesar had already stated the reasons behind that.
Darius had forgone organic growth for artificial growth. He could consciously choose to develop himself in any way, allocating his power where he wanted, as long as he gained the required Ability Points or Skill Points. According to his handler, Darius' growth speed was 100 times faster than the average Faustian.
Look at Jeanne. Darius might have complained about her talent, but he had no right to. She was a bonafide half-angel, so of course, her development speed would be crazy.
Angels only resided in Elysium, so it was a miracle that Jeanne's mother had managed to have had a liaison with one and carry a child. Lord knew what kind of plotting and suffering the woman had gone through to achieve that.