When the bug they're being forced to fix does not affect them. This can happen when they want to make some other change, or maybe they don't even need changes, they just need a build that doesn't exist.
New warnings usually do not indicate real bugs. Sometimes they do, but usually they are false positives. -Werror causing a build to fail is strictly worse than the alternative (working roughly as well as before, sometimes slightly better or worse due to new optimizations). This is the direct cause of "software decay" here. The software would not "decay" nearly as much without -Werror.
Often a technical linux user will want to download a source tarball for a particular version of a utility or program, and build it on or for a particular linux distro they're using. They usually do not want ad-hoc modifications done to the source of all the specific utility versions they compile for their system this way.
When that's the only way to just get the software, because there is no binary package. If it doesn't build or run, some users just give up and go onto something else.
Sometimes the package maintainers who make binaries for themselves and others don't want to fix bugs. At most some minor build things, but something actually broken is punted upstream.
You may see behaviors like the program being disabled (unavailable) for some platforms because it couldn't build or tests didn't pass. Or the package will stay on the old version of the program: 1.2.3 isn't building, so we just give up for now and stick with packaging 1.2.1. Maybe 1.2.4 will fix it. In these situations, package maintainers will not always contact upstream; the developer doesn't know unless they go into that distro's site and search the issues for activity related to their program.
