Alas... Demand for Users isn't constant.
Going into this assuming a constant 20 hour per day usage of every car in the fleet is called looking at things with rose colored glasses.
Knock that down to 6 hours a day, to have enough cars to accommodate the extremely busy times, and then idle a portion of the fleet waiting for the next call (probably by parking all over the city and powering up the closest car to the next fare).
Now you're down to $32,000/year.
Now subtract:
Insurance.
Gas or or power recharging
Inspections/Maintenance
Regular cleanings
Maybe you're looking at $15,000 per year after all that.
How much will a 4 person electric vehicle cost? $60,000? So that's 4 years til you see the first penny of actual profit. Unless you lease or borrow, then you're looking at financing costs/interest, etc. It'll allow you to see positive cashflow each month, at the cost of less actual profit over a longer period of time. Well, scratch that. No one is going to lease you the cars for the fleet - you're going to drive them into the ground by the time the lease is up. But Ubers big, they can obtain other financing either borrowing from banks or issuing bonds, for 4, 5, 6 or 7%.
And what will the useful life of these cars be? If they're averaging 30 mph for their fares, that's 63,000 miles per year. (not including other incidental mileage such as traveling to the next fare, returning for maintenance, getting a fresh charge, etc).
So after 4 years you've got a car with 250,000 miles. You got next to no resale value there. And this is when you start seeing actual profit from operating the vehicle (again if it was paid for in cash).
That's not so profitable anymore, is it?