Your OS’s own scripts use Python. So it installs Python by default. But the version of Python that it needs for those scripts, isn’t necessarily the same one that you want to use as a developer. Developers often want to run brand-new versions of open-source software, but the OS depends on stable versions of that same software.
The current solution is to just completely decouple the two. Your OS’s Python libraries are separate from the Python libraries you’re using as a developer. That way, you can update the latter to whatever unstable version you want, while being confident that you’re not going to break your OS.