I'm not the PC but I think you miss most of the pain points due to: 'personal' projects.

There's not a compatible format between different compilers, or even different versions of the same compiler, or even the same versions of the same compiler with different flags.

This seems immediately to create too many permutations of builds for them to be distributable artifacts as we'd use them in other languages. More like a glorified object file cache. So what problem does it even solve?

Because as a percentage of global C++ builds they’re used in probably 0.0001% of builds with no line of sight to that improving.

They have effectively zero use outside of hobby projects. I don’t know that any open source C++ library I have ever interacted with even pretends that modules exist.