Unreliable Service

Why does Uber hardly ever have available drivers? It's such an inconvenience especially if you rely on them for transport. but the majority of the time, on the app it requests a ride until it eventually says that there aren't any drivers available. They are there for those times when one does not have transport but how can you rely on them if they aren't providing the service they are supposed to be known for?? I don't understand it. You always need to have a plan B and can't really trust that Uber will be there when you need them.