I believe you have never worked for major tech company? In the company this size, they have multi product line (Uber, Uber Eats, Uber Freight, ATG etc) which are supported by their platform and infrastructure. It requires tons of eng work. Unlike smaller company seeks existing solution like open source, Uber needs to solve many of these themselves, which means build from scratch and also build tools to support it from scratch. Also, being the leader, you'll need to gain edge over competitors, e.g for Lyft or others, it’s sufficient to use Google Map or some other third party vendor as routing and ETA service, but Uber has its own Map team to improve the user experience. Hope this helps you understand