In principle that sounds great. In reality, that introduces more steps as each price offer and counter offer produces a time delay and makes me think.
Right now the app follows KISS (keep it simple stupid) and "don't make me think" principles.
A free market price negotiation would introduce a lot more complexity.
One way I could see this working is if driver's set their own rate (per minute or per mile) and riders can accept or reject that rate.