I use Uber or Lyft quite often (but not in NYC). I'd say that about 20% are foreigners/immigrants... certainly larger than the population at large, but there's no shortage of Americans (mostly college kids or older folks) who are doing the driving. It's not a terrible way to make extra disposable cash. Sit in your own car as much or as little as you want, only take the trips that you want, only work when you want, no boss to answer to, and make a decent "hourly wage" overall... listen to your radio, or enjoy light polite conversation with strangers for 10-20 min at a time... get paid faster than any "normal" job...
Most drivers enjoy the opportunity very much.
The demographic difference is probably geographic; you probably have more Americans around you.
I understand the appeal, but in the end they don’t make much money (factoring in time, wear & tear, insurance, and gas). It was funny when libs complained women were making less than men in these services; it is the purest “you work, you earn” environment (and indicated what may cause disparities in the larger workforce).