They are contractors because they do not have any set obligation for time. Rate of pay or overall pay has nothing at all to do with that. According to the IRS they do not have a set schedule and they are contractors (that is 1099, not W2).
So they are contractors. What does that have to do with pay? Absolutely nothing. You want x and are willing to pay y. Maybe there’s an opportunity for me to negotiate for z, maybe not. Either way none of that has anything to do with the actual form of employment. Even if I were a salaried employee working 40 hours a week the company has a budget of $40,000 a year and that’s all they will pay. If I demand $45,000 they’ll just tell me no - they simply don’t have the money.
In this case Uber offered you $x. You agreed to $x. You have no schedule, which is the perk of the job vs driving a cab, where you will have a schedule. Need to take the kids to school or a doctor’s appointment? Well, work that around your work schedule.
And as for your third point... Uber already charges me for both time and mileage. I don’t see any problem with them paying their drivers based on both... but from the drivers I’ve talked to they already do. I can’t speak for any other service at all.