Most of them do it only part-time and see it as a work-out gamification. Looks like during the pandemic many restaurants also closed delivery options and the orders dropped according to some YouTubers in the UK.
The most important aspect: It does not scale. It's very hard to earn a living outside of the "surge pricing" rush hours.
In some regions "surge pricing" is not available or delivery platforms require full time with a fixed but low salary (e.g. in Germany).
Here's a list of YouTubers doing bike deliveries:
- Chicago, IL (US) https://www.youtube.com/channel/UC4hE0AhisGkRscjxSMudn-Q/vid...
- London (UK) https://www.youtube.com/channel/UCruD32HAzsb9UN4KpOcSxZg/vid...
- Los Angeles, CA (US) https://www.youtube.com/user/impulsivewilliam/videos
- Bristol (UK) https://www.youtube.com/channel/UCZOtaF2WKa0wNysH4ZRH8-w/vid...