Just generally speaking, code that does orchestration and testing in general is often easier under a dynamic scripted language over something that is built and compiled, even if it winds up as a custom DSL. I think Python is one of the better options here for the broader community support, and tooling.

Aside: I tend to reach for node/js often for similar reasons (despite detractors) mostly because I'm more comfortable with it over Python or Ruby, but also because it's already integrated to most of the build/test environments I'm working on anyway.

Python is great for writing glue code, so I'm not particularly surprised by the language choice.

I’m curious why you seem to be surprised? It’s one of the most popular languages (even within Google).

That's one of the main languages at Google. And there are moving towards more Go since a few years.