Did you not see the other post which I quoted? Taxis are used more intensively than private cars, so they don't take up the same amount of central London parking space. That's the resource they don't use.
This isn't trivial. There are roughly 25,000 black cabs in London and they make roughly 200,000 paid journeys per day. (I would have thought it was more. But I guess quite a lot of them are long, eg from Heathrow.) If all those taxi journeys were instead made by private car, you'd need to find space to park an additional 175,000 cars in London. At 6m of kerb space each that would fill up 500km of roads, parked on both sides. That's about the same as London's entire network of Red Routes. Or in a car park it would require 3 sq km of land. There's not much "spare" land in London but to help visualise it, that's almost exactly the size of Kensington Gardens plus Hyde Park plus Green Park plus St James's Park combined.
I guess TfL decided that encouraging the use of black cabs by allowing them to use bus lanes would be a better solution than asking the Queen whether she'd be amenable to covering all those parks with tarmac.