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.