system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How do people in Urban societies in the Midwest, Northeast, South and West coast all have very similar accents/dialects.

Most black guys I know from Atlanta and New York sound just like me.
gpt
In the modern society, a lot of dialect becomes a result of broader influences than just your immediate geographical surroundings.  Television has standardized quite a few American dialects.  Most Americans watch network television which tends to use fairly standard dialects depending on their characters.

The standard media dialect, the type of dialect that people often refer to as "not having an accent" is very much reinforced in formal broadcasts like news programs.  This dialect is prevalent in the the midwest and west coast anyway, so there's a lot of it around.

Entertainment television and movies default to this dialect, too, but they'll have characters that they want to express a different regional identity, so they'll cast someone who has or can imitate a regional dialect, like for the movie "The Departed" where several of the actors had Boston accents (some more successfully than others).  If you wanted to cast some one for a character that's from the south, you'd likely have them use a southern accent, etc.

[edit] and to answer you question in regards to urban areas, they're the places that usually experience the most influence from television/movies/social media and which have the biggest mix of dialects, so someone living/growing up in an urban environment is more likely to end up with a standard dialect than any other particular one.

[edit 2]and the African American dialect is particularly standardized.  There is less variation among African American speakers than among almost any other major demographic.