It’s diversity of stuff that’s available. That’s what in my opinion differentiates DC from, say, Milwaukee or Even San Diego. You could do something different every day. It’s just that’s there is so much more options. I’m comparing to the places I’ve lived which have professionals sports (Milwaukee area, Detroit, and Research Triangle) where it’s worse than DC, and places I’m pretty familiar with that are better (Chicago).
This. It's not that other cities don't have theater, museums, historical attractions, etc., it's just that DC has much, much, more than most cities.
For example, did you know DC claims to have more theater seats per capita than any other city than NYC. May or not be true, but the point is that DC has a boatload of theaters as compared to, say, Indianapolis.
The only other cities that can compare with entertainment opportunities/options are NYC, Chicago, LA, and maybe SF. I know that's a debatable list and I'm sure I'm forgetting a city or two, but don't try to tell me that cities like Indianapolis, Milwaukee, St. Louis, Cleveland, etc., have as many entertainment options as DC.