So? Most of that is empty space nobody is trying to serve. The key metric is population density, which is comparable between urban areas in Europe and the US. The size of the US has nothing to do with lack of ISP choice, as evidenced by the fact that the same problem doesn't occur in countries with lower population densities than the US (33.8 people per km²) such as Sweden (21.5), Finland (16.2), Norway (13.4) or Australia (3.1).