Because they're trying to keep close together to conserve body heat? Uhm, no seriously... it probably has something to do with building code requirements and/or real estate economics. I'll have to look into the subject more, but my guess is that distances between structures, and things of that nature, are more heavily regulated here in the States, and it might be less expensive for a project to be placed in rim development (further out from a city's center) than core development. The problem we're facing here in Midwestern USA (the region west from Indiana to Kansas, south from Minnesota to Oklahoma) is that the cities are growing in terms of area, but are decreasing in density while doing so.
