Why Ubuntu doesn't ship latest mainline-long lived Nvidia driver?

If you want an operating system that provides packages for new software within hours, then Ubuntu is the wrong choice.

Ubuntu is focusing much more on stability, and in most cases even does not provide the newest version of software in older releases any more, see FAQ #3037: “no rolling release”.

Furthermore you have to be aware that Ubuntu copies software from Debian to avoid doiuble packaging work (from Debian stable or testing). As far as I can see, Debian has just added NVidia-450 in experimental (but only version 450.51). I assume it will take some more time until Debian has packaged 450.57 and has copied that from experimental into sid or bullseye. Only then Ubuntu will copy that package into its own development version (currently 20.10).

https://launchpad.net/~graphics-drivers/+archive/ubuntu/ppa

But Canonical maintains this ppa also and it doesn't exist there either. Which this ppa is opt-in to use.

PPA stands for Personal Package Archive.

Everybody can create a PPA and publish software. If you have a question about the contents of a specific PPA, then you have to contact the person or group who is responsible for that PPA. That is outside the scope of this question area.

In case that you want to search whether a specific software package is available in any PPA, then you can use the search function on https://launchpad.net/ubuntu/+ppas