Because they love other people
In the past year I had the privilege of visiting, among other places, Asilomar, CA; Austin, TX; Berkeley, CA; Berlin, Germany; Boston, MA; Davis, CA; Des Moines, IA; Las Vegas, NV; Los Angeles, CA; New York, NY; Niterói, Brazil; Ottawa, Canada; Pasadena, CA; Providence, RI; Rio de Janeiro, Brazil; San Francisco, CA; Seattle, WA; Victoria, Canada; and Washington, DC.
I feel very lucky to have been able to visit these great world cities, and, more often than not, to have been able to look around a bit and see a little of how these places differ from one another.
Only recently there was practically nobody who got to see all these places in a lifetime, and even now there is practically nobody who gets to see them in a single year, so I am incredibly fortunate.
Although most of these cities are mainstream tourist destinations, they have different kinds and degrees of marketing, and different self-concepts, and they rely on tourism to different extents. For example, I suspect Asilomar, Las Vegas, San Francisco, Victoria, Rio, and Niterói are especially dependent on visitors and have an unusually high proportion of their economic activity derived from outsiders' activity. Cities like Berlin, New York, and Los Angeles may have a much higher absolute volume of tourism than (say) Victoria, but it is hard to feel that the tourist industry is such a vital or essential force.
These cities also differ in other ways -- for example, in their levels of segregation along racial and economic lines. (San Francisco is enormously segregated in both respects, whereas Rio has even sharper economic divisions -- albeit compressed much more in spatial terms -- and astonishingly absent racial divisions.) The power of different groups and institutions is visible to different extents; Berlin struggles to show off its past, Victoria at least pretends to, and Los Angeles and Las Vegas often seem to rush to bury or redevelop it. In Ottawa you can see the power of the Canadian policy of bilingualism; in Victoria it frequently looks like a dead letter. Davis practices ecology in a way that puts Berkeley to shame; Las Vegas, um, doesn't. (Berlin, too, has an amazing ecological practice in its way, compared with American cities of a similar size.) New York has heard quite a bit from tort lawyers and Rio still seems to be awaiting them.
I don't want to claim some kind of great perspective on world events or cultures. In my whole life, I have only spent one week outside of a first-world country and only spent one weekend in the wilderness, sleeping outdoors. (I later spent a second weekend sleeping outdoors, but that wasn't exactly the wilderness since it had toilets, restaurants, commuter rail service, a supermarket, and the fastest Internet access I've ever had...) I am just grateful to have seen the variety that I have seen, which is very much focused on cities, the way my life is focused on cities.
I often like to quote Cicero's pessimistic view of human isolation, or rather the view he attributes to Africanus.
Vides habitari in terra raris et angustis in locis et in ipsis quasi maculis, ubi habitatur, vastas solitudines interiectas, eosque, qui incolunt terram, non modo interruptos ita esse, ut nihil inter ipsos ab aliis ad alios manare possit, sed partim obliquos, partim transversos, partim etiam adversos stare vobis; a quibus exspectare gloriam certe nullam potestis.
There are many aspects of this that remain true even in the age of air travel and global communications. I think it's a common essay question for people reading the "Somnium Scipionis" -- is this true even nowadays, that people live in little spots on the surface of the Earth and are so far cut off that nothing could pass between them? And students are meant to say yes, in a way. But I'm also thinking of how Dar Williams sang (perhaps a little ironically, if you recall about the line after):
People found the city because they love other people
