And of course we all knows the answer why: Cause these are the D&D rules. Part of its design.
Truly a very wierd design, which is sometimes a little immersion breaking.
Do not mistake Larian's unbalanced homebrew for D&D design.
https://www.dndbeyond.com/sources/basic-rules/combat#Droppingto0HitPointsIf Larian implemented the above section as written, you wouldn't feel it's broken.
"A stable creature doesn't make death saving throws, even though it has 0 hit points, but it does remain unconscious."
"Mighty villains and special nonplayer characters are common exceptions; the DM might have them fall unconscious and follow the same rules as player characters."
