You know the use case could be to make the transition easier. And since hyperion is using it so you could update it.
Until Kodi 19 uses Python 3 as a language interpreter, including Python 3 (as an add-on or not) would not help with the transition as Kodi would continue to use only Python 2 when executing Python add-ons.
Installing Python 3 into the image as well as Python 2 is a very niche requirement, which is why it hasn't been done so far. And for much the same reason nobody has submitted a Python 3 add-on as there isn't really the demand for it.
Also, the version of hyperion that is supported by LibreELEC 9.0 is using Python2. If someone wants to use a different and unsupported version of hyperion that has different Python language requirements then that is not our problem to solve.
And Cryologic has a use case. Package is already there.
@Crylogic hasn't explained what his use case actually is - he only asked why we don't include Python 3, which has now been explained.
And to my knowledge a package for a Python 3 add-on does not exist, but if anyone wants to submit it as a PR we can consider it, or better yet a community builder can build it/host it as it will have a very short life span.