I've heard that the reason this isn't the default is that parents get far more upset about making an elementary school kid get up and wait for a bus in the dark than a teenager.
I understand the arguments are made about the danger of standing next to a road invisibly (especially in shitty places with no sidewalks) but I'm sure some people are worried about kidnapping!
Heard that argument here when the idea was discussed. The funny thing is that these days (at least for Elementary school) there's always a parent or two who walks their kid to the bus stop and waits with them until the bus comes.