> I regularly have to work with other people's C++ where they don't have -Wall -Werror.

To be fair, -Werror is kind of terrible. The set of warnings is very sensitive to the compiler version, so as soon as people work on the project with more than one compiler or even more than one version of the same compiler, it just becomes really impractical.

An acceptable compromise can be that -Werror is enabled in CI, but it really shouldn't be the default at least in open-source projects.