It’s interesting that the OP didn’t also consider that people wear masks on bikes to block out truck and car fumes and other particles that tend to fly in your face. I rarely wear a helmet when biking, but there’s a noticeable benefit when I’m biking around traffic and don’t have to breath in car exhaust because I’m wearing a mask.
OP here - I was on the Stanford campus in 2016 and 2017, frequently riding my bike there. Never once saw a mask on a bike rider. Very little car traffic on the routes bicyclists regularly use to get around. Also a frequent bike rider in my new city of Dallas. Very rarely (if ever), have I seen a cyclist (even on roads) wear a mask. Very likely masks on the Stanford campus are in response to COVID.
It’s understandable that you wouldn’t see people wear masks on campus prior to COVID. Mainly because mask wearing outdoors was previously a rare event for any reason in the USA prior to COVID, so you’d stick out as making a statement if you were one of the few who did. But the normalization of wearing a mask outdoors now likely plays a contributing role towards people feeling comfortable about wearing masks for other reasons that aren’t as straightforward as ‘preventing COVID’.