I agree in Uber is not much use in central Tokyo because the taxis are so good and it's also easy to call a taxi (I generally call Kokusai) if you're not on a well traveled thoroughfare.

That said, I think there might be a market for longer fares if they offer competitive rates. Drivers typically get around 30k yen in fares per day. If they had an efficient way to do TYO<>NRT runs, they could charge much less and still make a lot more money than they do with normal driving.