Have you though? You're at A, near but not at the first bus depot. If there is a bus that originates at the depot, stops at A and then takes you all the way to B, you get there in 30 minutes. If it only takes you half way to B, you have to go half way, wait there for another bus from a different depot, take it back to that depot and wait for a third bus from the depot to B.

Not only is that slower, having the first bus stop there and turn around instead of going all the way to B hasn't benefited anyone else either. The route from the first depot to the halfway point is still being covered by the same bus route. The interval is determined by how many buses cover that route and not just by its total length, and another bus that covers that section of the route might even go somewhere other than B after reaching the point you'd have had it turn around to go back, also giving a faster option than multiple transfers for people who want to go from A to C. When A, B and C are all high density areas, creating direct routes between them makes sense even if they're each more than 15 minutes apart.