The problem with that scheme is that customers who tip generously in cash (that the driver then doesn’t declare) appear to be just the same as customers who tip poorly.
The reason that isn't done is 1) bad tipping has some "demographic" components, and the companies don't want to be accused of discrimination, and 2) the driving company gets their fee regardless of whether the drivers get tipped. I road along with my son when he was doing this one summer, and you can see the trends fast. But knowing that your customer has been highly rated by other drivers would be huge.