Destiny is essential for any life form, especially as they grow stronger and face even deadlier challenges. It could be considered the sum of their talent, hard work, willpower, life experiences, and many more factors. A slight improvement could mark a massive rise in one cultivation limit and survival skills.
However, tempering it was challenging even for Primarchs and could have disastrous consequences. Improving someone's destiny was not something so simple as raising their soul or body talent.
For example, suppose Zatiel or Ezequiel had monitored and decided every step in Heinz and the others' cultivation. In that case, that may have granted them a solid foundation but would not have improved their destiny. On the contrary, it would have crippled it.
The idea that they could not tackle essential decisions regarding their cultivation without help would have risen in their souls, severing their path to the peak.
