You put in a different depot for the 20 minute trip. With express buses the trip between the two depots may only be 5 minutes.
People through history have always considered about half an hour a reasonable daily commute. Doesn't matter if it is a hunter-gatherer going to their gathering grounds (if they follow herds they will move camp if the herd moves more than half an hour), or "modern man" going to the office, half an hour is what you get. Everything I said is based on making as many of those half hour trips possible as I can - but not all trips can be done that way and some locations will be left out.
You would then be subjecting people to indirect paths with multiple transfers. Instead of going from A to B you'd have to go from A to the first bus depot to the second bus depot to B, waiting for another bus at each transfer. If the trip was 30 minutes to begin with, now you've made it 45 (at best).
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.
