Normally, curses are designed to amongst other things, make the afflicted weaker in some way to make sure that they have less of a chance to retaliate against you.

However, that’s not the case with the curse Celestia put on Khaenri’ah and possibly other civilisations.

Yes, they become hideous monsters unable to die, but it also makes the cursed stronger. They are able to manipulate elements which is something normal humans cannot do, and can grow into Lawachurls, or become Abyss Lecters amongst other things which are far more powerful than almost any human can hope to naturally become.

So, Celestia probably isn’t cursing people just as a form of punishment, but rather to create enemies and dangers that makes humans rely on them and justify their iron fisted rule.

Afterall, if you don’t have a threat to rally your hapless citizens behind, they are more likely to question why they need to suffer under your regime and start rebellions.

So Celestia went straight for the 1984 solution of manufacturing dangers and wars to keep everyone under control.