Quote Originally Posted by Dualight View Post
The PHB even has a section on this.
The Monster Manual also establishes this as a default rule, while the PHB couches it as something "most DMs" do.
Quote Originally Posted by Monster Manual, page 7
A monster usually dies or is destroyed when it drops to 0 hit points. For more on hit points, see the ' Player's Handbook.
Quote Originally Posted by JonBeowulf View Post
In short, because PCs are special and nothing else is. Functionally, it's so players have a safety net when things don't go their way.
Basically this. DSTs are a mechanic meant to relay the metagame concept that the PCs are special. This means, among other things, that despite the memes that circulate around, that a DM who has "intelligent monsters double tapping downed PCs" is metagaming.