Having more than one job doesn't mean I'm necessarily self employed. I could work at a supermarket during the mornings, and a cornershop in the evenings, and still be an employee in both cases.
There's a paradox in this: Uber and its competitors would ideally like their drivers to be 100% committed; yet they also need the drivers to be able to work for several apps as a defense against requalification as an employee.
I mean there are even a number of worker supply firms out there that provide workers on this basis. A business needs people to clean floors, they issue a request to their supplier and they supply the nearest, cheapest available workers.
Only difference is the length of the shift, and the amount of notice before hand.
Many supermarkets only provide shift schedules hours before they start. Uber just provides their shift schedules seconds before they start.