Have you ever looked at an airport and immediately blamed the Airport Editing Team for not adding a ground or approach frequency? I can assure you that they did not forget to add that approach frequency. Probably. While governments are inefficient, they aren’t about to pay a controller to watch over 3 gates. Because of this, airports with small simple ramps often don’t include a ground control. The same is true of approach control in less flown areas. Pilots are known to be afraid of mountainous regions and will avoid them when possible. This lower amount of traffic isn’t enough to get an approach control added. In these cases, the center controller will act as approach.