A combination of growth rate, and positing where Uber will be once they stop spending money on things like self-driving cars, etc, which are very expensive with

A combination of growth rate, and positing where Uber will be once they stop spending money on things like self-driving cars, etc, which are very expensive with no revenues yet. It's similar to Amazon that for years only had minimal profits because they were spending so heavily on new products. The idea was that Amazon could generate tons of money by simply curtailing their spending, which fuels the speculation on their share price.