Man this industry is crazy. Most traditional companies fight hard to gain even 1% in net margin.
dotcom v3.0 companies are all about the potential and cornering the market. Amazon was exactly the same - it was founded in 1994 but didn't make a profit until 2001.
There's no reason they need to be spending such crazy amounts on servers - ostensibly to allow faster iteration. A new version of their app just isn't going to move the needle. Signing up new drivers will though.
I fail to see how the benefits AWS provides are so important they need to spend such crazy amounts.
Every extra driver costs them money, every lift costs them money. If they can hold out the illusion of "we know we can save money here when we've got time and have won the market", maybe it keeps the money rolling in.