Keeping piles of cash in your vehicle is not completely safe. Just think of everything what can potentially happen to a driver, riding around with all the money of the day.
No more looking for change coins is, by the way, also pretty!
Storing money on a bank account is simply more comfortable and more safe.
People tend to pay higher tips when they pay with a credit card. It is a common psychological tric. When you pay by card, you get to see the three common tips options in the first place (15%, 20% or 25% of the ride price), and only then the possibility to enter the tips amount manually.
A tiny example. Say, a ride costs $17.90. While paying with cash, most people will give a driver $20. In case they tip with a credit card and choose the cheapest option (15%), the driver will receive $20.15. Not exactly all the money of the world, but multiply it by the number of rides per day...