Leaving game-world history lore aside, for a moment...
Lore, typically, speaks in generalizations - it presents stereotypes. Insisting that stereotypes apply to all specific individuals seems, in terms applicable to your personal preferred style, like questionable simulation.
You get to want what you want, of course. But, in the context of trade offs, "must abide by the GM's views on details of my character's behavior," would probably look like a notable trade-off to lots of players.
A great example is how ethnically and culturally diverse even real-world rough historical analogues were. Travelers travelled, often away from their homes for years at a time (I was just reading an account of an African-homeported Jewish merchant and philosopher who floated and caravened off to India for like 4 years, coming home to a now toddler son). Divergence happened between one valley and the next, depending on who’d migrated in and intermixed over the preceding so many years.
Our fantastic settings are often far less diverse and interesting in a cultural and beliefs level than real life. I was just reading an interesting blog post talking about how to make a setting that more resembles that level of diversity, focused entirely on humans/mono-species.