if you read the docs for this hook you'll understand why the default is the way it is

Where are the docs you're referring to? I had already read the docstrings in https://github.com/pre-commit/pre-commit-hooks/blob/main/pre_commit_hooks/sort_simple_yaml.py and they all talk about YAML files, there is no mention that this hook is meant for other filetypes.

the README is typically the place to look first

Note that sort-simple-yaml by default matches no files as it enforces a very specific format. You must opt in to this by setting files

I see what happened: Whoever set this hook up in the monorepo where I hit this did so via files: ^our_team_subdirectory/ and did not include any extension matching in that regex (e.g. [^.]+\.ya?ml$ at the end), so it ended up unintentionally rewriting .py files. Perhaps this person made the reasonable assumption that this hook was already limiting itself to yaml files.

I already fixed the regex in our repo (before even submitting this PR). I am still wondering if it makes sense to accept this PR – would including the types: [yaml] here not make it easier to use this hook? And match the docstrings better, which indicate the hook is only intended to target yaml files?

no it would make it more difficult in the case that you used this on a file which identify doesn't recognize as yaml -- and usually files is sufficient in this case.