Nor do you for Lyft/Uber. You're simply looking for work for both simultaneously.

I'd say the only way to "simultaneously" work for both is to have a rider from more than one service in the car at the same time. Probably 0% chance they'd have an overlapping route, so you'd have to go off-route for one of them, and I'm sure that wouldn't go well. But for all the drivers I've seen that use more than one service to find riders, never seen this happen.

I don’t see a difference between shift work at multiple super markets and multiple ride for different ride hailing apps.

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.

Just curious, when are you "working" for uber or lyft? I'm honestly not sure how it works. Do you collect an hourly wage when you are waiting? Or do you only collect money when you actively driving a passenger?

That's a limitation of humanity, not a property of employment.