I think the reasons languages like Zig choose not to make this configurable is because other than the odd exception occasionally very few shops bother to turn on the stricter mode or are actively hostile to the stricter mode.

And when you rely on other libraries that sort of less strict mode tends to start leaking out everywhere since that one critical library you really don't want to rewrite won't compile when you use strict and you don't want to carry a bunch of patches for it either.

The net result is if you don't start out enforcing cleanliness your community of users will unintentionally devolve into unclean over time.