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There are. But, they are inherently limited. Via fixed supply and fixed pricing.

Exactly what is Limited by supply and price? Surely not the app? Perhaps the dispatcher charges a fee for access to the API?

The cabs themselves. There are a limited number of licensed cabs, and prices are preset.

I don't see how that would negate the convenience of an app.

You need a variable supply to accomodate for varying demand. Uber would not be able to deliver 10min and under pickups if their supply was limited.