Because if something works, and the actual project developers are comfortable with it, why change it? For new projects, people may prefer to use something like

Because if something works, and the actual project developers are comfortable with it, why change it? For new projects, people may prefer to use something like Zig as the build system. But a build system is not what will make the project faster or magically add new features and fix bugs. What makes that happen is developers being comfortable with the source code, including its build system.