Nothing except the fact that there seems to be an extreme taboo that makes everyone steadfastly refuse to do it with the same fervor.

If it were really the only thing giving Uber a monopoly on the market I guarantee somebody else would lose their scruples.

Tipping isn't about scruples. Businesses with scruples would pay their employees a fair wage and price accordingly. And then immediately go out of business as everyone flocks to the competitor that's magically 20% cheaper (before the essentially mandatory 20% tip).

Tipping is entirely about being able to advertise a lower price than the actual customer cost. Unfortunately that is extremely effective, so if one company in a market accepts tips all the others basically have to.

It's not like Uber was paying a generous wage in the first place.