Why that $14 billion can not be called revenue is Uber specifically structures it's business to make sure their drivers form no part of the business, instead they insist all their drivers are independent contractors.
They do that specifically to reduce their running costs and hence maximize the profit on their actual revenue.
That would be like saying all iPhone App developers are working for Apple, when in fact all Apple does is take 30% of each and ever dollar these App developer earn in sales.
Likewise Uber just puts it's 30% Uber tax on every dollar it's drivers earn.
$14 billion is revenue
$3 billion is their earnings after driver costs.