I can’t tell you why that is in general, but I can talk about the specific case of why I did for the bike bus. I could see a similar benefit for adults too (like a casual bike-pool), but adults can go directly from A to B at their own pace, and leave at their own time, they have disparate workplaces, can better mind their own safety and be more visible on the road than kids. So the bike bus I see as way more beneficial for kids getting to school, than for adults going to work or other places.
I can’t tell you why that is in general, but I can talk about the specific case of why I did for the bike bus. I could see a similar benefit for adults too (like a casual bike-pool), but adults can go directly from A to B at their own pace, and leave at their own time, they have disparate workplaces, can better mind their own safety and be more visible on the road than kids. So the bike bus I see as way more beneficial for kids getting to school, than for adults going to work or other places.