This is more like gypsy cabs. Uber/Lyft don't have to ply the streets b/c of their app and backend tech.
The Uber app specifically tells drivers how to ply the streets - what direction they should be driving in order to maximize their chances of picking up a profitable fare in the future. They call it "surge pricing".
Do you mean that Uber drivers can be hailed?
Inside the app you can see them plying around you and you just hail them in your phone. The interface is much more practical than hailing a taxi using your hand.