It's because they're trying to make a module system that is compatible with decades of development based around `#include`. It's nowhere near as simple as making a new language with modules.

I almost feel like they should give up. IMO Rust and Zig have made C/C++ a legacy language. Except in some specific areas where Rust doesn't have mature libraries (GUIs & games mostly) you'd be silly to start a new project in C++.

Migrating existing C++ projects to modules likely isn't worth the enormous effort, and the days of starting new C++ projects are numbered... so why bother?