If someone designed a setting for Level Up that was modeled after Ancient Greece, chances are the Culture portion of the setting would be modeled after a particular Greek City-state. So you would have your Sparta-like culture, and your Athens-like culture. There wasn't one culture for the entirety of Ancient Greece. Ditto for Medieval France or even Modern Day America.
My two issues then are:
1. This can still be handled by player choice rather than mechanical intervention. My Athenian PC might want to represent his culture by taking diplomacy, but he might also want to be a proud warrior of Athena or a cynical mercenary who doesn't care about the art of politics. Ditto a Spartan who due to his frailty never learned how to fight, but was an amazing tactician and logistics person. I don't have the need for him to have medium armor and intimidation proficiency.
2. The thinner you slice up culture, the more unique types you need to make. A setting like Eberron, even with everyone having been part of a continent wide empire a century ago, would need cultures for the various nations of Khorvaire, Sharn, Cyre Refugees, The Demon Waste tribes who fight for and against demons, the various elven tribes, the Dhakaani nations, the Dragonmarked Houses, Lhazaar Pirates, etc. I could probably see 30 or more for just Khorvaire, not even counting the continents beyond. That's a lot to keep track of and make unique mechanical expressions for, and it still runs the risk of "Every orc or other person from the Shadows March gets a druid cantrip of their choice" style homogenization.
I get it's a fix to give elves the ability to use longswords and bows as trait again by giving them "Evermeet" as a culture that grants it, but it still feels kinda unnecessary since if I want to have that ability, I can just select those options. To each their own.