All the startups clumped under the heading Uber for X—of which there are dozens—require high concentrations of people (i.e., cities). Take companies like Airbnb, which works best with a lot of listings in a given area, or Yelp, which declines in utility as fewer people contribute reviews. But take, as well, the range of other technologies that make the most sense within dense agglomerations of people. The most prominent example is autonomous vehicles, which require detailed, expensive, and regularly updated maps to operate. For that reason, those vehicles will almost certainly deploy in cities first, and maybe only in places where enough people drive to make the investment in mapping the area worth it.
Cities are also where developers generally want to live—so if you wanted the best people to build some other kind of technology, you’d need to go there to find them.