There is just no need to. It's small, so the overhead of fetching it is minimal. Once fetched it can be cached practically indefinitely and re-used by any site

There is just no need to. It's small, so the overhead of fetching it is minimal. Once fetched it can be cached practically indefinitely and re-used by any sites that uses the same CDN, while still making live updates possible. Bundling it would only provide a stale version, add bloat and open up a floodgate for other inclusions.