I'm not sure, but the discussion I remember was that it would require changes to an awful lot of places in the docs that would make the docs harder to read.  It is very seldom in normal Python coding that an object has a method that it does *not* get from its class, and by the time you get to the level where you are adding methods to instances it is likely you know about dunder methods only being looked up on classes.  Usually you learn about it the first time you try to put a dunder method on an instance, and you scratch your head for a while, and then you find out...but complicating the docs to mention this at every turn would be...excessive, I think.  But again, I'm not 100% sure I'm remembering the outcome of that conversation, and I can't remember where it took place (it was an issue in this tracker, but I don't know which one.)