I think that's because NYC is one of the few places that an app just like you describe would make any sense. :)
In many places, the list of "things happening this hour on a weekend afternoon within 10-20 minutes of where I am right now that I might possibly be interested in" approaches zero.
On the other hand, if there is a huge list of events happening close to you soon at any given time, the problem is "that I might possibly be interested in." The "things my friends expressed interest in" is useful not just because my friends might be there (although that can be a plus, or a prompt to text one of em and say hey you wanna go to this), but because if a buncha my friends are interested in it, that makes it more likely it will be something that will appeal to me, because my friends in aggregate are more likely than random to be interested in things I am.
I'll give you another scenario. I was in Mississippi (virtually the opposite of NYC) with my sister and her 2 children. We wanted to find things to do in the area that was kid friendly but also relatively cheap.
This led me to spending something like 30-45 minutes trudging through various websites, the local paper, etc. I eventually found about 3-5 different options.
Simply broaden the definition of "things happening" and the number is definitely not zero.
In many places, the list of "things happening this hour on a weekend afternoon within 10-20 minutes of where I am right now that I might possibly be interested in" approaches zero.
On the other hand, if there is a huge list of events happening close to you soon at any given time, the problem is "that I might possibly be interested in." The "things my friends expressed interest in" is useful not just because my friends might be there (although that can be a plus, or a prompt to text one of em and say hey you wanna go to this), but because if a buncha my friends are interested in it, that makes it more likely it will be something that will appeal to me, because my friends in aggregate are more likely than random to be interested in things I am.