>>95876148
It has something to do with the health economy. In games where you are expected to have a fight or maybe two a day, on top of potential hazards and traps draining health, it would lead to players being extremely cautious with these types of mechanics.
Furthermore, JRPGs tend to have way more rounds of combat, which in actual game time is also a lot quicker. You are expected to take damage constantly and spend many rounds hitting enemies, especially in boss fights. That also means you get dedicated healer characters to heal a lot, or use items to heal. Meanwhile, in an actual ttrpg, healing during combat doesn't happen constantly, and it's often at times when things went horribly wrong. And you usually get a lot less healing (less items, less healings spells to cast).
To make this actually worthwhile, you will probably need to make the buff from sacrificing health really good, but this could mean that the dark knight just one-shots encounters. Afterwards, he replenishes all his health during the rest, because what's stopping him?
I really want to make dark knights like this work in a weeby rpg, but writing all this...