Think of all the random bugs you’ve seen in your job and told yourself “eh, this would take someone 2 weeks to fix and is almost never hit by customers”
Basically, once you hit scaling I think you end up with a super small team that managed to keep simple (instagram for example) or you end up with a huge team that explodes in complexity and needs to grow exponentially to handle all the extra complexity. Uber is very obviously in the later bucket.
Let me assure you, it is a BFD if I get billed twice for the same trip. So I am pretty sure Uber needs quite a few engineers to make sure that their stuff works correctly every time in every market for every customer.