There's an excellent HN thread that talks about this very question (that comes up on HN every now and then - what _does_ company X do that needs so many engineering resources?): https://news.ycombinator.com/item?id=25375921

TL;DR: Managing a taxi service (that's what Uber is in my mind, not whatever "ride share" means) that spans cities and states, never mind countries, is extremely complicated. To their credit, Uber manages to make it look simple to the end user, prompting such comments as "meh it's just a few screens how hard could it be", which is triumph of product engineering as far as I am concerned.

Related: this blog from Uber talks about the problem of serving market-specific configuration data at scale: https://www.uber.com/us/en/blog/how-we-unified-configuration...