@ed because, as explained in the issue, people (and corporations) compile and use open source software from source routinely, and even more in these years of supply chain attacks.
A failing build can be fixed in an hour (at most, if the build process is properly documented), but a personal data-leak can not be fixed at all after it occurred.
People who can't find DNS server more suited for their needs can happily use Google's one, but it must be a conscious decision not something they are not aware of.
As for it being a systemd responsibility, I think it is (they wrote that code and picked the worst possible defaults).
And frankly, they happily renamed the default branch from `master` to `main` to please US people who do not like to remember their #slavery heritage, why they shouldn't do a cheap fix here that benefit all the rest of the world?
Also note that **no actual distro build would be affected** as they already define the constants.
So why not?