Maxperson
Morkus from Orkus
It's actually in the 5e rules. None of them in the PHB or DMG are about NPCs to PC. All of them are PC to NPC and the designers confirmed that they are not supposed to be used on PCs.I have to agree. There's no ironclad reason social skills can't work on PCs. Just common (but not universal) custom.
Now, the DM can absolutely change that, but by default they aren't supposed to be used against PCs.