> I can understand keeping one stable LTS branch and one "up to date" branch

If you did that then lots of distro packagers would just stay on the LTS until the next one comes along, and so you'd increase the number of users that are on older versions. With the selected approach, there is more work involved to backport a fix to the 4-5 supported versions, but this gives the benefit that OS distros can bump their Python versions more frequently, and so end users get updated Python3 features more often.

All Linux distributions and MacOS ship with Python, and many users of Python stick to the OS-provided version without installing the latest and greatest on their own.

For example, they might only upgrade major Python versions (3.x) once they migrate from Ubuntu 16.04 LTS to 18.04 LTS or even directly four years to 20.04 LTS. Whichever Python version is available by default there becomes the next Python LTS for them.

For now, all versions since 3.4 receive occasional security and bug fixes to make all of them are "stable". New features are only adopted with a major version (3.x) bump.

I have not heard of the Python community complaining about this being too much work - after all improvement work is only done in the latest version, old versions just get backported bug fixes.

I guess you aren't missing anything. They do security releases for 5 years after the release of a minor version:

https://devguide.python.org/devcycle/#security-branches

Arbitrary, but not senseless or anything.

If you drill into the security releases, they are pretty small patches backported from later releases, not a huge burden.