User blog comment:IamJakuhoRaikoben/Chapter 358: Magical Barrier Particles, Review./@comment-5036235-20131101050854/@comment-4829046-20131101132737

The homunculi in Full Metal Alchemist were immortal too but they still had a limited number of lives or rather, times they can regenerate until their immortality is up. Same for most series. No matter how powerful you are, you can't come back to life after getting destroyed countless times in a row. Even though your body is renewed, your spirit isn't. Eventually you yourself would have had enough with regenerating at every point in the fight and would just stop altogether.