I think the point is that if they were more expensive (and that money went to drivers), then users might think twice about summoning a car when they don't really need one, and give more consideration to more traffic- and environmentally-friendly alternatives. There's also the possibility that drivers might be able to make a reasonable living without feeling pushed to cut corners in maintenance, rest, etc.

Just giving tips to drivers can help out the individual drivers who benefit from them, but it doesn't shift the balance for the ecosystem as a whole.