Because the game hates "healing" :

- one shot = healers are powerless
- vulnerability stack, can't be removed = healers are powerless
- QTE = healers are powerless
- "KO on the 2nd strike" = healers are powerless

And the root cause is, the devs hate players' agency ; proved continuously since ARR. They don't want us to go against their design, and healing is fundamentally about turning a situation around (healing the raid-wide aoe at the start of a fight isn't really "healing", to make things clear)