with your logic, there is no such thing as homebrew because the DM can run the game however they like.
Originally Posted by DMG pg 4
...the rules aren't in charge. You're the DM, and you are in charge of the game
Anything and everything the DM does is RAW
On a more serious note, there's a difference between explicitly spelled out scenarios in the rules (e.g., how attack rolls work) and things which are explicitly left up to the DM (e.g., giving Advantage for circumstances or setting DCs for traps/to persuade NPCs).
If a DM set all persuasion checks to DC 30, that'd be a bad gameplay decision and frustrating for the players. But it'd still be technically following the rules. This is equally true for setting all persuasion checks to DC 1.
