Homogenization of culture is one of the effects of maintaining an empire. I'd argue an intentional one, as it makes people easier to rule if they aren't ever exposed to new ideas. It happened in Rome, too, albeit through different pathways than in the U.S.

You listed a bunch of cities, that's where the effect of homogenization is strongest. The last bastions of real culture are out in the country, that's where traditions can be maintained independent of an empire's archonic stranglehold. Internet hasn't completely squashed them all out yet, but even those are strongly affected now.