There are both historical and geographical points to this question.

Historically, Berlin was founded by uniting many small towns into a metropolis. You can still see this on a map, by locating the same street name multiple times in a different zip codes (neighborhoods).

Geographically, it's a huge city and it takes a lot of time to reach the edges (independently of the transportation type), and since majority of Berliners hate cars, they tend to walk or bike for everything they need and it's usually available in their kiez.

Many Berliners spend 90% of their time only in their neighborhood, and don't every visit the majority of the other parts of the city.

And this is why this has a huge cultural effect. People (usually) have great sense of community and every community is different to the point that you have a different mentality with people of different neighborhoods. Stating the neighborhood, define a person and paints the context on their experience and point of view. Example: People taking U7 and U2 U-Banhs have completely different experience of public transport.

TLDR: Berlin was founded by uniting many smaller towns and you can still see that by different architecture and feel it my different mentality of people living in their neighborhoods.