Google has 180,000 employees. There's an org that is responsible for internal tooling, but they are an org with priorities and challenges like any other. So sometimes Search needs a thing and Core won't do it so some people in Search build a tool. Maybe later somebody in Ads needs a similar tool but has never heard of the thing that Search built. So there is an organic force to this sort of complexity.

There are systems to resist this and people whose whole job is to focus on unifying systems, but they aren't perfect. In general, I'd say that Google is way way way more uniform than most other companies because of the monorepo, centralized tooling, and (mostly) uniform development process across the company.