The Uber app does not include a tip when billing you for a trip fare.
In most cities, Uber is a cashless experience. Tipping is voluntary. Tips are not included in the fare, nor are they expected or required.
As a rider, you are not obligated to offer your driver a gratuity in cash. If you decide you would like to tip, your driver is welcome to accept.
Where available as a vehicle option, uberTAXI is an exception. uberTAXI connects riders with licensed yellow cabs, and includes the option to set a gratuity percentage added to your trip fare.