What is the reason distros are still building from release tarballs rather than a git checkout that can be verified against a public git repo?

code repository are not necessarily git based. Plus you would need to put the effort in monitoring the activity of the repository for changes.

Until last month, would you refuse a tar package from the official maintainer, I wouldn't, especially when there was a mention of a bugfix that might have been tripping our build system

For example nginx is using mercurial (with admittedly a github mirror for convenience), and a lot of OSS are still using subversion and CVS, and my guess is that there are some project which might run with less free source control software ( most likely for historical purpose, or use case that might be the strong point of that software).

Other than that, why wouldn't the user be the one to build their own software package.

I think a lot of it is probably historical. When debian or red hat infrastructure came up there was no git; projects were still often in source control during development but tarballs were still the major distribution mechanism to normal people. Though before git they'd sometimes have packages that would be based on an SVN or cvs snapshot back in the day, in absence of releases.

I believe what happens in debian is that they host their own mirror of source tarballs, since going to some random website or git repo means it could be taken down from under them. So I guess if the package is built straight from a repo they'd probably make a tarball of it anyway.

There could potentially be many things you would not want to commit to git. Binary files and generated files come to mind. There could also be transformations of code for performance or portability reasons. Or ones that require huge third party dependencies that are only used to the build script.

There are many potential reasons to publish a release tarball where some of these steps are already done. It could be done in a reproducible way. Look at sqlite for an example of an extremely well maintained open source library that publishes not one but two source code tarballs for various stages of the build.

These calls to change source code distribution just because it was a small part of the attack vector in this particular case seems misguided to me. It may still be a good idea but only as part of a much larger effort for reproducible builds. In itself it would accomplish nothing, apart from a wake of uncertainty that would only make future attacks easier. Especially in this case, where the maintainer could have acted in a number of other ways, and indeed did. The entirety of the backdoor was added in a regular git commit a long time ago.