Please note that no passport is required between Paris and Berlin either
That's a relatively recent change.
I would assert that the cultural distance is thousands of times that of Boston to the West Coast.
And that's a surprise in what way?
And by the way, the cultural distance from Cambridge, MA to Montague, MA is far greater than the cultural distance between Cambridge, MA and Berkeley, CA.
one can go from the Slavic end of the Balkans to North Africa in a couple of hours, for instance.
Thank you for making my point. Since folks here in the US can't do that, due to geography, we have a far lesser need for passports. Therefore, it is understandable that many people in the US don't have passports.