There are but Uber has the best combination of price/coverage/lack of randomly refusing certain destinations. It also works internationally and really helps get over the language barrier when travelling.
Many minicab firms only pick up within certain areas around their office, apart from Addison Lee who charge an arm and a leg.
Black taxi availability is highly variable outside of the centre especially late at night, and then you run into "not going to xyz/south of the river this time of night mate". Their fares get expensive very quickly, although over short journeys they actually compare well to Uber especially as the drivers aren't just mindlessly following an app and can pick the best route.
Uber are a reprehensible company but they have a fantastic product. I just wish they charged a bit more but invested it into safety and driver welfare.