I don't follow your logic. 

It's highly common for restaurants to pool tips across the staff. If everyone provides great service, everyone gets great tips and vice versa. This obviously creates ambiguity for any staff individual to know the exact amount each customer tipped but, even if they did, it wouldn't change anything. The restaurant still has to strive for excellent service across all customers if they want to maximize their tips. 

This is the same principle for drivers. If you deliver late, forget a bag, or make the customer meet you outside, then you should expect little to no tip. I.e. Suboptimal service = suboptimal tip. 

What would a driver do different if they knew what each customer tipped?