The largest cost will be in sales and marketing. They offer large incentives to get drivers to sign up, such as offering cash for becoming an Uber driver. They also want to provide the best price possible to consumers, so that sometimes that means selling below cost, or at least selling below the cost when all sales and marketing expenses are included.
Growing so quickly in a short period of time there is also typically some level of over staffing as well as generally not being 100% diligent with use of funds.
There was an article before how they were spending over $100MM with an ad agency only to realize the numbers were inflated and eventually dropping them, but that is just one example of ineffective spend.
That's not to imply that they are 50% ineffective, but even being 10% ineffective when budgets are in the billions adds up.
The thought process is that the cost of switching between providers is very low when it comes to Uber/Lyft, when the cost of switching is low to consumers you want to establish your dominance in that market as rapidly as possible otherwise if you move slowly, then a competitor can steal market share away from you.
If there are any network effects to market penetration then that begins to create a bit of a moat. So if you think about the availability of cars/drivers at any time of the day to meet the demand, then having a large marketshare is beneficial to getting a ride for a customer quickly. That couple with marketing and lowering the cost of the ride as much as possible to establish the market is where the majority of that money goes.