Around the country, using these whenever I travel, rarely have I encountered obvious foreign or immigrant workers. Most are clearly lifelong US drivers, mix of mid- and sometimes older drivers. Most notable observation is that most are new to driving for Uber.
I’m in northeastern NJ, where half the fares are foreigners as well (Americans are steadily evacuating the area). The types of foreigners driving here are similar to those that work office-cleaning jobs at night, but these are a little better off (disciplined enough to buy and insure a car).
Young Americans can’t afford the car to begin with, and older ones couldn’t afford to live here on the wages paid by these services.