No, they simply don't allow drivers to work for competitors with a simple trick: designated routes. Working within the same day on different apps is not enough, it has to be different apps => multiple clients => one trip. If an Uber driver were to pickup a Lyft user on a trip and to drop them off away from the main algorithmicly imposed road. Uber would warn the driver within a few days, regardless of client reviews.

I was not clear enough: indeed, they can't drive simultaneously different clients (from different apps). Instead, while they are waiting for a new client, they have several apps open, and if they get ride propositions from different apps, they pick the most lucrative one.

I don’t see why the same couldn’t apply to supermarket employees, where you can dynamically choose your shifts in real time and pick the highest of all the available work.

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.