It's how it got started, and now undertaking is regarded as a normal practice when riding a bicycles, much like riding on pavements.
Up until 14 I was always told to stop riding in the road and stay on the pavement, then I was told to stop riding on the pavement and start riding in the road and sit in the gutter out of the way, Which I still get shouted at me, aka that I shouldn't be more than a meter from the edge of the road.
Luckly I was never really one for doing as I was told but a lot of people will only have ridden bikes as children, where they were taught to ride on the pavement or edge of the road. so that's what they will expect
Up until 14 I was always told to stop riding in the road and stay on the pavement, then I was told to stop riding on the pavement and start riding in the road and sit in the gutter out of the way, Which I still get shouted at me, aka that I shouldn't be more than a meter from the edge of the road.
Luckly I was never really one for doing as I was told but a lot of people will only have ridden bikes as children, where they were taught to ride on the pavement or edge of the road. so that's what they will expect