I think one of the big reasons why I'm into homebrew settings instead of established ones is due to the information base--especially for settings like Forgotten Realms, Dragonlance, Star Wars, Marvel/DC, Lord of the Rings, etc. Though there are some players who may have knowledge about the game to understand certain nuances & reference, not all of the players may share that level of knowledge.
And to a certain degree, some players emulate elements of an established setting a bit too much, whether it's a prominent NPC (Elminster, Drizzt, Raistlin, etc.), a unique race (draconians, saurials, thri-kreen, etc.), or even unique abilities/concepts (spellfire, Chosen of XX deity, wild magic, etc.). It gets a bit tiresome after a while.