No reason in particular. Mostly because I’m not super familiar with CircleCI and wasn’t sure if bringing in the Pulumi Docker image as an executor would cause problems for most workflows.
e.g. would that cause problems if you want to perform your build/test steps on another image? Or can files (e.g. the ./src/bin directory) be shared seemlessly between different “executors”?
If you have opinions and/or insights please do file an issue over at github.com/pulumi/circleci. Happy to work on making things “more correct” and useable here!