Uber (the company) likes people to think that, so that they can tout their "cashless" transactions. They can no longer say it explicitly, after a lawsuit by a group of drivers.
In reality, your Uber driver is not receiving a gratuity from Uber, and is probably netting less than a typical taxi driver, even before tips. But that's the cost of setting your own schedule and driving as much or as little as you like, so it's entirely up the rider whether they choose to tip. The vast majority of UberX riders do not tip.
I'm not too familiar with Uber Black, but I know they are independent professional drivers, so I would be uncomfortable if I used them and did not tip.