Where are you getting the 80% number? The drivers I talked to have told me it's much lower.
driverDollars = (fareDollars - 1)*0.8
If a driver is mostly taking people one or two blocks, the fares are going to be in the $3-5 range. That means the $1 "safe ride fee" (or whatever it's called) is already 20-30% of the fare, so Uber's total take on short rides is closer to 50%. It's one of the reasons drivers don't like short rides.
[1] edit: By net I mean after driving expenses, not after Uber's take.