Uber isn't part of the service economy; it's part of the gig economy. The former warrants a tip, whereas the latter does not. According to Vox's Aditi Shrikant:
Unlike cab drivers, Uber or Lyft drivers are not technically in the service industry. Although Uber drivers are providing a service, they make their own schedule and are not beholden to a company that pays below minimum wage. Being in the gig economy is a choice they can opt out of at any time.
Although workers in the gig economy likely face similar financial struggles to those in the service industry, only 35% of riders are likely to tip their Uber drivers, and millennials are less likely to tip. Shrikant explains a few reasons why riders are not inclined to tip:
A rider who doesn’t tip says it’s because when the app was introduced, tipping wasn’t an option, which seems to signal that tipping is not an expectation. If it wasn’t a priority then, why should it be a priority now?
Something else the second non-tipping rider said was that she has stopped taking Ubers and Lyfts because she doesn’t like that a portion of her fare goes to Lyft and Uber, and not back into her local economy as it would if she took a cab. And this idea of a non-local, impersonal service may be why people see tipping as optional.