Most people who ride bikes have no idea how to shift gears (maxmautner.com)

there must be a strong causal link with terrain / the nature of these rides. I live in hilly country so I learned as a kid how to upshift and downshift. 80% of the rides I take would be slow and laborious with a single-speed bike. I'm willing to bet most cheap single-speed bike people (of those who aren't using them out of financial necessity or social conformity) are urban riders who have mostly flat terrain to cover.

I didn’t learn how to use gears until I took up road cycling last year. I remember walking my bike up every hill when I was a kid