Cells have both a maximum range and a _maximum population_. The multiplexing that enables many devices to use the same spectrum to communicate with a tower is limited, so you can't just put up one tower in Manhattan and you're done, that'd fill up with users immediately, you need to add lots of towers in areas with many simultaneous users or the service is lousy. As a bonus the reduced transmission distance means handsets (automatically) reduce radio power, which means improved battery life.

As a result cells are naturally much smaller in populated areas where you're most likely to want detailed information. In a city centre the cells may be only a hundred meters across. True, up a mountain the cellular network may have no idea where on the mountain you are, but this isn't a Safety of Life application, it's a survey.