Windows doesn't have bash preinstalled so I'm not sure why you're bringing that up. On all OSes I've used in the past 3-4 years all of them have been Python3 by default. The specific version is largely irrelevant as I could ask you the same question about which version of bash is running on a given OS (e.g. it's not available on Mac out of the box as the default shell is zsh due to GPLv3).