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.

Most engineers create tools, Google is not special in that way.

But even when there's a similar open-source tool, it may be difficult to integrate with internal systems. There's a lot of benefit to a consistent set of technologies (data formats, storage systems, etc) so that's often a reason to write internally.

There is a lot of open source software in use at Google so I wouldn't say there's a lot of reinventing the wheel. And when there is an internal clone, the requirements are often necessarily different enough to warrant writing an internal version.

I did not interpret their statement as meaning they have many variants of the same tool, but that they have tools that cover a wider variety of situations.

I found the same thing moving from a very large company to a smallish company. Both had good tooling, the latter just had significantly less and there were gaps in coverage. Whenever I encountered one of those areas I often found myself wishing I had access to some niche tool I had gotten used to at my prior company.

In many cases there was no wheel available at the time