Why do you expect that’s going to be hard to implement? Actual private contractors (myself included) do that all the time.
One straightforward method, a rider posts location + destination + price of the ride, accepts/declines offers from drivers based on price, vehicle model, reputation, personal preferences, etc.
Another straightforward method, a passenger posts his location + destination, drivers offer price, a passenger accepts an offer.
> The value added by these platforms is exactly that
Not “exactly that”, that’s just one part of it. Examples of e-bay or stock markets shows that an intermediary connecting buyers with sellers, processing payments, maintaining reputation of both parties, and solving disputes, can still add huge value.
> you don't need to negotiate with individual drivers to get a fare to your destination
With sufficient support from the service, the negotiation part can probably be streamlined to only take a couple of clicks/swipes. Like preference to filter drivers/riders by price or distance, ability to save routes or destinations with price information allowing for 1-click repeated orders, other convenience features.
It’s technically possible, and not that hard, to implement a voice-based interface for that, using e.g. Bluetooth + car stereo.
You setup your preferences while the vehicle is not moving to filter the offers you want to receive, by criteria like price/km, geography, min. total price. Then the software uses voice interface to read you the incoming offers, and listens to your “accept/decline” voice commands.
The value added by these platforms is exactly that, you don't need to negotiate with individual drivers to get a fare to your destination.