But then shouldn't they be profitable already? How are they losing multiple billions per year if they're profitable per unit.

If the answer is they need to raise prices and/or reduce the drivers' cut, how will they be better than the taxi services they're replacing? Many taxi services up here in Canada already have apps that are close to on par with what Uber offers.

i don't know how did you reach the conclusion of costs amounting to 'pennies', they are a taxi service without owning or leasing a taxi (generally) and without investing into depreciating assets, does not make the costs 'pennies' even worse, their one time investment might not be higher but monthly operational and running expenses are far more therefore, who hires taxi drivers, and it's almost costing them more than they pay to taxi 'partners' and all other costs involved, hence the massive losses, have you got a car? go to their office, and 'recruit' yourself and do the math.

pssst!! it's not a 'angelic ride sharing' service period. it's hiring taxi drivers and in this case mostly entrepreneurial taxi drivers having multiple cars and running it as a business.

And yet, they're losing billions. Those Cassandra servers don't run themselves...