While that is true it is a simplification. There are providers that service only the main populations such as FREEDOM (was WIND). However many people decided against these because they have to pay extra when leaving the city, such as visiting a cottage or going camping. So there is at least
some market demand for providing wide coverage included in your base plan.
(I don't really understand it though, because Freedom Mobile has agreements to roam on the other networks and while the cost is much increased, for people who leave the city once every month or two it doesn't add up to the cost of the more expensive providers. These days Freedom even includes some included roaming in their plans, presumable to help customers see this line of thinking.)