Just dealing with the different laws about taxi services in different countries/cities itself would require a staff that size or more. And there's a lot more than that behind the scenes, like security checks on drivers, managers for each city to decide incentive plans for drivers etc. It's a very massive endeavour, and the size of the company is justified.
It takes a constant number of engineers to push a single feature, no matter the number of users.
It does not, however, take a constant number of engineers to maintain the infrastructure that lets a single engineer push a feature to millions of users. (P/I)AAS can help, but you still need people to monitor performance, find regressions and bugs, and track them down.
