Head over to
ISP-Plant and click through the links of backbone providers and you'll see a trend emerge -- they all peer and build capacity in a handful of cities around the world.
In the US the capacity is cheap between these locations for one reason: the railroads. Since the railroads own all of the land immediately adjacent to the tracks that cross the continent, the major telcos / fiber providers were able to negotiate laying fiber with just a handful of major railroad companies to get contiguous right-of-way for large distances. Go out to any major railway and you'll see the distinctive orange fiber markers along the route. See: »
ops.fhwa.dot.gov/freight ··· ange.htm for railway maps.
When they have to trunk capacity out beyond those locations, they have to work to get rights from all the land owners in path and every municipality wants their cut. The administrative overhead to work out contracts with all the entities to run fiber within a metro area is huge.