As I explained on another comment of mine (https://news.ycombinator.com/item?id=41409199), it's not since their beginning, but since a particular zlib security incident which showed them the risks of vendored libraries. Any discussion of vendoring policy which does not consider that incident is necessarily incomplete.
The distro people could've been working on the tooling for automating this at the distro level (and some people in debian are doing work that would enable this); we don't need to go this insane "unbundle everything" route.
IMO this is a huge mistake in the Linux distro world. It doesn't scale. It's similar to the weird idea that Debian should contain every program in the world and if the authors want their programs to be installable on Debian they must make a Debian package for it. And Fedora and Arch and Gentoo and...
Clearly bonkers.
They, and most others, have been doing that since their respectively beginnings, and for good reasons: everybody expects distros to fix security issues, which is greatly aided by ensuring an-as-small-as-necesary dep tree and thus a single version of any particular dep.
Changing that isn't impossible, but I am not aware of any distro that's managing this and be widely considered 'stable'.
How can distros change while keeping up their end which is stability and robustness in the face of bugs and security issues?