On the other hand, Amazon continued to improve operating margins and expand offerings which led them to profitability. Every indication is that Uber is selling

On the other hand, Amazon continued to improve operating margins and expand offerings which led them to profitability. Every indication is that Uber is selling rides cheaper than they cost - and the things they can do to reduce cost also disincentivize their drivers. Secondarily, Amazon was public for years - which means they were required to become profitable. Uber doesn't yet have that pressure in place- and a new round of cash delays that requirement even further.