Culture is important to character.
It is what your PC's bonds, ideals, flaws, and personality are built from.
So if your cultures are shallow or your setting has few cultures, then the PC's personalities are limited.
How so? How do you suppose then that
writers manage it when they tell a story set within the bounds of one culture? Why,
their characters don't even have
stats to distinguish one from another…
An all samurai campaign would be boring as heck to me. You'll all be human fighter samurais with the same ideas, bonds, and traits. Just a different flaw.
I don't play an edition that uses ideals, bonds, flaws, etc. Mechanizing that sort of thing strikes me personally as a ghastly notion. But supposing that I did… why in the world would you imagine that they'd all have to be the same for every member of a party of samurai?
Culture is not character. Do you think that in an Arthurian setting, Lancelot and Gawain and Galahad have all the same personality traits because they're all Arthur's knights? That in an Ancient Greek setting, Achilles and Odysseus and Agamemnon (who are all human fighters who share a culture, a language, a pantheon) are somehow
similar characters!?
I do have a problem with the elitist view that curated campaigns are inherently better than kitchen sink games. And I've had a huge problem with some of the reasons why people have given to omit content.
For the record, you can replace "curated campaigns" with "low-magic" "grim-n-gritty", "open world sandbox" or "decade old homebrew world" as things people tend to view as superior and act like they play D&D better or more correctly than the unwashed masses.
The prevailing view on the DM's side has been that
we have a right to our curated campaigns, and you don't get to call us tyrants for having them. We've further demonstrated that there's no such thing as a campaign that isn't curated, in practical terms. Even the most wide-open kitchen-sink has character types that won't fit and must be disallowed, and boundaries (however soft) on what rulebooks are considered canonical.
And for the record, the only time I've seen anyone on this thread claim objective superiority, it's been when the likes of doctorbadwolf and loverdrive make sweeping claims that (a) "the hobby" has "grown out of" the DM being an authority over their campaign, and (b) that this is an objectively good thing. Both claims are highly suspect at the very least (and they positively
reek of elitism—lesser in degree but similar in kind to that same elitism that one can detect around zealous players of GM-less storygames who love to crow about how they've thrown off the "shackles" of D&D).
The "DM partisans," from what I've seen, have been fairly consistent in saying
do it your way, while a certain few of the "player partisans" have said
do it our way or you're a bad DM.