That is a result of 2 mechanics.
1) Help action restoring your HP. Raw you should only be able to get a party member up by feeding them a potion or using a spell, which are limited resources.
2) Action economy (this one isn't Larians falt). Creatures that are designed to be bosses tend to have legendary actions (ability to act more then ones per round).
The Medicine skill in 5e allows for stabilising a dying character, which effectively puts them in the state of being unconscious but not needing to make Death saves. The Healer's kit also gives this ability.
Neither gives them 1hp and gets them walking again, though.
