What Uber is in this context is basically an intermediary, like a retailer that sits between a manufacturer and a consumer, or a general contractor that sits between a subcontractor and a property owner.

What you have then isn't a transaction between the driver and the rider, it's two transactions, one between the driver and Uber and another between Uber and the rider. Each transaction is negotiated. Uber changes their offers to each party based on supply and demand, and if either party doesn't like it they can hold out for a better offer or go use Lyft. And since Uber does actually adjust pricing based on supply and demand, the better offer actually comes when enough people hold out.

The way you negotiate with them is by delaying your purchase/sale until they meet your price. It's a functioning market.