There are periods of incredibly high demand, ie late on a Fri/Saturday night. There are times when the service is brought into peak demand unexpectedly, ie, subway line breaks down, metro strike. There are times when demand is low but a taxi service is still basically a necessity (trips to the airport at 4am).
Taxi driving never should have been a full time business. It was decreed somewhere that it was a full time “job” but that is incredibly uneconomical. When there’s no demand for your typical taxi, your car sits around and depreciates on it’s own while having to cover the high costs of basically running a “business”, licensing insurance etc.
Uber was made to be a part time job because Taxi driving is best suited for that naturally.