To some extent there are, but in many cases it's because the law prohibits that. It imposes build out requirements so that if you want to offer service in the higher density area you also have to provide service over a wide area that includes a bunch of farmland or similar.
In theory this is supposed to be to make sure someone is serving rural areas but in practice it's because the incumbents don't want new challengers showing up to provide better service in the areas where it's profitable to build a competing network and so lobby the government to saddle them with a huge barrier to entry.
In theory this is supposed to be to make sure someone is serving rural areas but in practice it's because the incumbents don't want new challengers showing up to provide better service in the areas where it's profitable to build a competing network and so lobby the government to saddle them with a huge barrier to entry.