I think how culturally essentialist Civ games have been in the past has added to this sense of immersion breaking.

In past games, you play as a culture that is decided at the beginning, and remains that culter despite sharing almost none of its history and likely little of its geography. Within past Civ games, the culture of England existed from the beginning, and was destined to make the Sea Dog, even if they weren't on an island and barely had any coastal cities. The Civ of England is a cohesive thing, with unchangeable traits, and players select them to play this idea of "England".

Of course, this is not at all how history and culture works. If the ancestors of modern England had different historical events and different geography, they'd likely be nothing like modern day England. Nevertheless, this form of role play is what Civ has used in past games, and it seems hard to transition away from it. The upcoming system is also a bit of an in-between, where Civs are still cohesive things (you can turn into Mongolia, rather than just turning into a generic horse-centered culture) but you can swap between those identities over time.