Public buses are not like airplanes, where groups of people board and exit the plane at the same location. One person may see a bus coming into Coupeville, as an example, completely full, and another person will note that same bus leaving Coupeville with only a few, or no people. By the time that bus arrives in Oak Harbor, it is completely full.
People get on and off the bus all over the county.
Also keep in mind that there are times throughout the day when the buses are driving out of service, which means they are in the process of a shift change, are heading back to the Island Transit bus barn for the evening, or to be swapped out for maintenance purposes. The smaller buses are not manufactured with diesel engines at this time. Cutting Island Transit mid-day service would like closing a restaurant between lunch and dinnertime. It just does not make sense from an economic or service standpoint.