Is it? I've seen zero people wearing them in central london.

I haven't noticed anyone wearing them in real-life, despite walking every day in a concentration of university/college campuses (Harvard Square, USA, and surrounding neighborhoods, sometimes including MIT).

I've only noticed some influencers wearing them in Web videos, where they look prominent, like I should also be able to recognize them in-person.

Maybe people are only wearing them at home or in the office? Maybe it's mostly a California techbro regional thing?

Maybe I'm not paying enough attention. I will start consciously looking for them.